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Trapezoidal Rule

Mathematics · FE Reference Handbook section

Mathematics
15 formulas
10 exam-style examples
~60 min
All Mathematics lectures

Core formulas for this FE topic

Definitions, applicability, units, assumptions and worked examples for each relation.

Worked exam-style examples

The four ways this section is written on the real exam — thoughts first, then equations, then substitution.

Example 1
Trapezoidal rule for numerical integration — solve for approx. integral — Trapezoidal Rule

An engineer applies the trapezoidal rule to approximate the area under a velocity curve. Given step size (h) = 1.4000; f(x0) (f0) = 7.4000; f(x1) (f1) = 5.7000; f(x2) (f2) = 5.9000, determine the approx. integral (I).

Given

  • stepsize(h)=1.4000step size (h) = 1.4000
  • f(x0)(f0)=7.4000f(x_{0}) (f_{0}) = 7.4000
  • f(x1)(f1)=5.7000f(x_{1}) (f_{1}) = 5.7000
  • f(x2)(f2)=5.9000f(x_{2}) (f_{2}) = 5.9000

Find

approx. integral (I)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except I is given, so isolate I symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 1 — schematic for Trapezoidal rule for numerical integration — solve for approx. integral — Trapezoidal Rule

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for I:

    I=h2[f0+2f1+f2]I = \dfrac{h}{2}\left[f_0 + 2f_1 + f_2\right]
  3. Step 3

    Listthegivens:stepsize(h)=1.4000,f(x0)(f0)=7.4000,f(x1)(f1)=5.7000,f(x2)(f2)=5.9000List the givens: step size (h) = 1.4000, f(x_{0}) (f_{0}) = 7.4000, f(x_{1}) (f_{1}) = 5.7000, f(x_{2}) (f_{2}) = 5.9000
  4. Step 4 — Substitute the given values:

    I=1.40002[f0+2f1+f2]I = \dfrac{1.4000}{2}\left[f_0 + 2f_1 + f_2\right]
  5. Step 5 — Evaluate:

    I=17.2900I = 17.2900
  6. Step 6 — Check: returning I = 17.2900 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
I=17.2900I = 17.2900

Why the other options are there

  • 34.5800 — kept a factor of two that cancels in the correct rearrangement.
  • 8.6450 — dropped that same factor in the other direction.
  • 19.0190 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 2
Trapezoidal rule for numerical integration — solve for step size — Trapezoidal Rule (2)

A student uses the trapezoidal rule to estimate an integral from tabulated data. Given f(x0) (f0) = 4.8000; f(x1) (f1) = 1.0000; f(x2) (f2) = 3.0000; approx. integral (I) = 100.8, determine the step size (h).

Given

  • f(x0)(f0)=4.8000f(x_{0}) (f_{0}) = 4.8000
  • f(x1)(f1)=1.0000f(x_{1}) (f_{1}) = 1.0000
  • f(x2)(f2)=3.0000f(x_{2}) (f_{2}) = 3.0000
  • approx.integral(I)=100.8approx. integral (I) = 100.8

Find

step size (h)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except h is given, so isolate h symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 2 — schematic for Trapezoidal rule for numerical integration — solve for step size — Trapezoidal Rule (2)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for h:

    h=I(f0+2f1+f2)/2h = \dfrac{I}{(f_0+2f_1+f_2)/2}
  3. Step 3

    Listthegivens:f(x0)(f0)=4.8000,f(x1)(f1)=1.0000,f(x2)(f2)=3.0000,approx.integral(I)=100.8List the givens: f(x_{0}) (f_{0}) = 4.8000, f(x_{1}) (f_{1}) = 1.0000, f(x_{2}) (f_{2}) = 3.0000, approx. integral (I) = 100.8
  4. Step 4 — Substitute the given values:

    h=100.8(f0+2f1+f2)/2h = \dfrac{100.8}{(f_0+2f_1+f_2)/2}
  5. Step 5 — Evaluate:

    h=20.5714h = 20.5714
  6. Step 6 — Check: returning h = 20.5714 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
h=20.5714h = 20.5714

Why the other options are there

  • 41.1429 — kept a factor of two that cancels in the correct rearrangement.
  • 10.2857 — dropped that same factor in the other direction.
  • 22.6286 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 3
Trapezoidal rule for numerical integration — solve for f(x1) — Trapezoidal Rule (3)

The trapezoidal rule is used to approximate the flow volume from measured readings. Given step size (h) = 2.8000; f(x0) (f0) = 4.4000; f(x2) (f2) = 2.0000; approx. integral (I) = 47.7300, determine the f(x1) (f1).

Given

  • stepsize(h)=2.8000step size (h) = 2.8000
  • f(x0)(f0)=4.4000f(x_{0}) (f_{0}) = 4.4000
  • f(x2)(f2)=2.0000f(x_{2}) (f_{2}) = 2.0000
  • approx.integral(I)=47.7300approx. integral (I) = 47.7300

Find

f(x1) (f1)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except f1 is given, so isolate f1 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 3 — schematic for Trapezoidal rule for numerical integration — solve for f(x1) — Trapezoidal Rule (3)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for f1:

    f1=2Ih−f0−f22f_{1} = \dfrac{\frac{2I}{h} - f_0 - f_2}{2}
  3. Step 3 — List the givens: step size (h) = 2.8000, f(x0) (f0) = 4.4000, f(x2) (f2) = 2.0000, approx. integral (I) = 47.7300.

  4. Step 4 — Substitute the given values:

    f1=247.73002.8000−f0−f22f_{1} = \dfrac{\frac{247.7300}{2.8000} - f_0 - f_2}{2}
  5. Step 5 — Evaluate:

    f1=13.8464f_{1} = 13.8464
  6. Step 6 — Check: returning f1 = 13.8464 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
f1=13.8464f_{1} = 13.8464

Why the other options are there

  • 27.6929 — kept a factor of two that cancels in the correct rearrangement.
  • 6.9232 — dropped that same factor in the other direction.
  • 15.2311 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 4
Trapezoidal rule for numerical integration — solve for approx. integral (case 2) — Trapezoidal Rule (4)

An engineer applies the trapezoidal rule to approximate the area under a velocity curve. Given step size (h) = 1.5000; f(x0) (f0) = 3.9000; f(x1) (f1) = 4.9000; f(x2) (f2) = 3.2000, determine the approx. integral (I).

Given

  • stepsize(h)=1.5000step size (h) = 1.5000
  • f(x0)(f0)=3.9000f(x_{0}) (f_{0}) = 3.9000
  • f(x1)(f1)=4.9000f(x_{1}) (f_{1}) = 4.9000
  • f(x2)(f2)=3.2000f(x_{2}) (f_{2}) = 3.2000

Find

approx. integral (I)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except I is given, so isolate I symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 4 — schematic for Trapezoidal rule for numerical integration — solve for approx. integral (case 2) — Trapezoidal Rule (4)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for I:

    I=h2[f0+2f1+f2]I = \dfrac{h}{2}\left[f_0 + 2f_1 + f_2\right]
  3. Step 3

    Listthegivens:stepsize(h)=1.5000,f(x0)(f0)=3.9000,f(x1)(f1)=4.9000,f(x2)(f2)=3.2000List the givens: step size (h) = 1.5000, f(x_{0}) (f_{0}) = 3.9000, f(x_{1}) (f_{1}) = 4.9000, f(x_{2}) (f_{2}) = 3.2000
  4. Step 4 — Substitute the given values:

    I=1.50002[f0+2f1+f2]I = \dfrac{1.5000}{2}\left[f_0 + 2f_1 + f_2\right]
  5. Step 5 — Evaluate:

    I=12.6750I = 12.6750
  6. Step 6 — Check: returning I = 12.6750 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
I=12.6750I = 12.6750

Why the other options are there

  • 25.3500 — kept a factor of two that cancels in the correct rearrangement.
  • 6.3375 — dropped that same factor in the other direction.
  • 13.9425 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 5
Trapezoidal rule for numerical integration — solve for step size (case 2) — Trapezoidal Rule (5)

A student uses the trapezoidal rule to estimate an integral from tabulated data. Given f(x0) (f0) = 8.6000; f(x1) (f1) = 5.3000; f(x2) (f2) = 7.4000; approx. integral (I) = 53.3900, determine the step size (h).

Given

  • f(x0)(f0)=8.6000f(x_{0}) (f_{0}) = 8.6000
  • f(x1)(f1)=5.3000f(x_{1}) (f_{1}) = 5.3000
  • f(x2)(f2)=7.4000f(x_{2}) (f_{2}) = 7.4000
  • approx.integral(I)=53.3900approx. integral (I) = 53.3900

Find

step size (h)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except h is given, so isolate h symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 5 — schematic for Trapezoidal rule for numerical integration — solve for step size (case 2) — Trapezoidal Rule (5)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for h:

    h=I(f0+2f1+f2)/2h = \dfrac{I}{(f_0+2f_1+f_2)/2}
  3. Step 3

    Listthegivens:f(x0)(f0)=8.6000,f(x1)(f1)=5.3000,f(x2)(f2)=7.4000,approx.integral(I)=53.3900List the givens: f(x_{0}) (f_{0}) = 8.6000, f(x_{1}) (f_{1}) = 5.3000, f(x_{2}) (f_{2}) = 7.4000, approx. integral (I) = 53.3900
  4. Step 4 — Substitute the given values:

    h=53.3900(f0+2f1+f2)/2h = \dfrac{53.3900}{(f_0+2f_1+f_2)/2}
  5. Step 5 — Evaluate:

    h=4.0143h = 4.0143
  6. Step 6 — Check: returning h = 4.0143 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
h=4.0143h = 4.0143

Why the other options are there

  • 8.0286 — kept a factor of two that cancels in the correct rearrangement.
  • 2.0071 — dropped that same factor in the other direction.
  • 4.4157 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 6
Trapezoidal rule for numerical integration — solve for f(x1) (case 2) — Trapezoidal Rule (6)

The trapezoidal rule is used to approximate the flow volume from measured readings. Given step size (h) = 1.5000; f(x0) (f0) = 5.6000; f(x2) (f2) = 2.7000; approx. integral (I) = 84.2500, determine the f(x1) (f1).

Given

  • stepsize(h)=1.5000step size (h) = 1.5000
  • f(x0)(f0)=5.6000f(x_{0}) (f_{0}) = 5.6000
  • f(x2)(f2)=2.7000f(x_{2}) (f_{2}) = 2.7000
  • approx.integral(I)=84.2500approx. integral (I) = 84.2500

Find

f(x1) (f1)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except f1 is given, so isolate f1 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 6 — schematic for Trapezoidal rule for numerical integration — solve for f(x1) (case 2) — Trapezoidal Rule (6)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for f1:

    f1=2Ih−f0−f22f_{1} = \dfrac{\frac{2I}{h} - f_0 - f_2}{2}
  3. Step 3 — List the givens: step size (h) = 1.5000, f(x0) (f0) = 5.6000, f(x2) (f2) = 2.7000, approx. integral (I) = 84.2500.

  4. Step 4 — Substitute the given values:

    f1=284.25001.5000−f0−f22f_{1} = \dfrac{\frac{284.2500}{1.5000} - f_0 - f_2}{2}
  5. Step 5 — Evaluate:

    f1=52.0167f_{1} = 52.0167
  6. Step 6 — Check: returning f1 = 52.0167 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
f1=52.0167f_{1} = 52.0167

Why the other options are there

  • 104.0 — kept a factor of two that cancels in the correct rearrangement.
  • 26.0083 — dropped that same factor in the other direction.
  • 57.2183 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 7
Trapezoidal rule for numerical integration — solve for approx. integral (case 3) — Trapezoidal Rule (7)

An engineer applies the trapezoidal rule to approximate the area under a velocity curve. Given step size (h) = 0.8000; f(x0) (f0) = 5.7000; f(x1) (f1) = 1.5000; f(x2) (f2) = 4.4000, determine the approx. integral (I).

Given

  • stepsize(h)=0.8000step size (h) = 0.8000
  • f(x0)(f0)=5.7000f(x_{0}) (f_{0}) = 5.7000
  • f(x1)(f1)=1.5000f(x_{1}) (f_{1}) = 1.5000
  • f(x2)(f2)=4.4000f(x_{2}) (f_{2}) = 4.4000

Find

approx. integral (I)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except I is given, so isolate I symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 7 — schematic for Trapezoidal rule for numerical integration — solve for approx. integral (case 3) — Trapezoidal Rule (7)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for I:

    I=h2[f0+2f1+f2]I = \dfrac{h}{2}\left[f_0 + 2f_1 + f_2\right]
  3. Step 3

    Listthegivens:stepsize(h)=0.8000,f(x0)(f0)=5.7000,f(x1)(f1)=1.5000,f(x2)(f2)=4.4000List the givens: step size (h) = 0.8000, f(x_{0}) (f_{0}) = 5.7000, f(x_{1}) (f_{1}) = 1.5000, f(x_{2}) (f_{2}) = 4.4000
  4. Step 4 — Substitute the given values:

    I=0.80002[f0+2f1+f2]I = \dfrac{0.8000}{2}\left[f_0 + 2f_1 + f_2\right]
  5. Step 5 — Evaluate:

    I=5.2400I = 5.2400
  6. Step 6 — Check: returning I = 5.2400 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
I=5.2400I = 5.2400

Why the other options are there

  • 10.4800 — kept a factor of two that cancels in the correct rearrangement.
  • 2.6200 — dropped that same factor in the other direction.
  • 5.7640 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 8
Trapezoidal rule for numerical integration — solve for step size (case 3) — Trapezoidal Rule (8)

A student uses the trapezoidal rule to estimate an integral from tabulated data. Given f(x0) (f0) = 6.8000; f(x1) (f1) = 4.1000; f(x2) (f2) = 5.9000; approx. integral (I) = 82.6200, determine the step size (h).

Given

  • f(x0)(f0)=6.8000f(x_{0}) (f_{0}) = 6.8000
  • f(x1)(f1)=4.1000f(x_{1}) (f_{1}) = 4.1000
  • f(x2)(f2)=5.9000f(x_{2}) (f_{2}) = 5.9000
  • approx.integral(I)=82.6200approx. integral (I) = 82.6200

Find

step size (h)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except h is given, so isolate h symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 8 — schematic for Trapezoidal rule for numerical integration — solve for step size (case 3) — Trapezoidal Rule (8)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for h:

    h=I(f0+2f1+f2)/2h = \dfrac{I}{(f_0+2f_1+f_2)/2}
  3. Step 3

    Listthegivens:f(x0)(f0)=6.8000,f(x1)(f1)=4.1000,f(x2)(f2)=5.9000,approx.integral(I)=82.6200List the givens: f(x_{0}) (f_{0}) = 6.8000, f(x_{1}) (f_{1}) = 4.1000, f(x_{2}) (f_{2}) = 5.9000, approx. integral (I) = 82.6200
  4. Step 4 — Substitute the given values:

    h=82.6200(f0+2f1+f2)/2h = \dfrac{82.6200}{(f_0+2f_1+f_2)/2}
  5. Step 5 — Evaluate:

    h=7.9062h = 7.9062
  6. Step 6 — Check: returning h = 7.9062 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
h=7.9062h = 7.9062

Why the other options are there

  • 15.8124 — kept a factor of two that cancels in the correct rearrangement.
  • 3.9531 — dropped that same factor in the other direction.
  • 8.6968 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 9
Trapezoidal rule for numerical integration — solve for f(x1) (case 3) — Trapezoidal Rule (9)

The trapezoidal rule is used to approximate the flow volume from measured readings. Given step size (h) = 2.4000; f(x0) (f0) = 7.1000; f(x2) (f2) = 9.7000; approx. integral (I) = 59.9200, determine the f(x1) (f1).

Given

  • stepsize(h)=2.4000step size (h) = 2.4000
  • f(x0)(f0)=7.1000f(x_{0}) (f_{0}) = 7.1000
  • f(x2)(f2)=9.7000f(x_{2}) (f_{2}) = 9.7000
  • approx.integral(I)=59.9200approx. integral (I) = 59.9200

Find

f(x1) (f1)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except f1 is given, so isolate f1 symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 9 — schematic for Trapezoidal rule for numerical integration — solve for f(x1) (case 3) — Trapezoidal Rule (9)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for f1:

    f1=2Ih−f0−f22f_{1} = \dfrac{\frac{2I}{h} - f_0 - f_2}{2}
  3. Step 3 — List the givens: step size (h) = 2.4000, f(x0) (f0) = 7.1000, f(x2) (f2) = 9.7000, approx. integral (I) = 59.9200.

  4. Step 4 — Substitute the given values:

    f1=259.92002.4000−f0−f22f_{1} = \dfrac{\frac{259.9200}{2.4000} - f_0 - f_2}{2}
  5. Step 5 — Evaluate:

    f1=16.5667f_{1} = 16.5667
  6. Step 6 — Check: returning f1 = 16.5667 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
f1=16.5667f_{1} = 16.5667

Why the other options are there

  • 33.1333 — kept a factor of two that cancels in the correct rearrangement.
  • 8.2833 — dropped that same factor in the other direction.
  • 18.2233 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

Example 10
Trapezoidal rule for numerical integration — solve for approx. integral (case 4) — Trapezoidal Rule (10)

An engineer applies the trapezoidal rule to approximate the area under a velocity curve. Given step size (h) = 1.8000; f(x0) (f0) = 6.2000; f(x1) (f1) = 1.2000; f(x2) (f2) = 7.7000, determine the approx. integral (I).

Given

  • stepsize(h)=1.8000step size (h) = 1.8000
  • f(x0)(f0)=6.2000f(x_{0}) (f_{0}) = 6.2000
  • f(x1)(f1)=1.2000f(x_{1}) (f_{1}) = 1.2000
  • f(x2)(f2)=7.7000f(x_{2}) (f_{2}) = 7.7000

Find

approx. integral (I)

Start with the thinking

  • The governing relation printed in this handbook section is Trapezoidal rule for numerical integration.
  • Everything except I is given, so isolate I symbolically first — never rearrange after the numbers are in.
  • Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
  • The trapezoidal rule approximates a definite integral by summing trapezoid areas between panel points.
xyTrapezoidal rule area under curve

Figure 10 — schematic for Trapezoidal rule for numerical integration — solve for approx. integral (case 4) — Trapezoidal Rule (10)

Step-by-step solution

  1. Step 1 — State the governing relation:

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]
  2. Step 2 — Rearrange symbolically for I:

    I=h2[f0+2f1+f2]I = \dfrac{h}{2}\left[f_0 + 2f_1 + f_2\right]
  3. Step 3

    Listthegivens:stepsize(h)=1.8000,f(x0)(f0)=6.2000,f(x1)(f1)=1.2000,f(x2)(f2)=7.7000List the givens: step size (h) = 1.8000, f(x_{0}) (f_{0}) = 6.2000, f(x_{1}) (f_{1}) = 1.2000, f(x_{2}) (f_{2}) = 7.7000
  4. Step 4 — Substitute the given values:

    I=1.80002[f0+2f1+f2]I = \dfrac{1.8000}{2}\left[f_0 + 2f_1 + f_2\right]
  5. Step 5 — Evaluate:

    I=14.6700I = 14.6700
  6. Step 6 — Check: returning I = 14.6700 to

    ∫abf(x) dx≈h2[f(x0)+2f(x1)+f(x2)]\int_a^b f(x)\,dx \approx \dfrac{h}{2}\left[f(x_0) + 2f(x_1) + f(x_2)\right]

    reproduces the given quantities, and both sides carry the same units.

Answer:
I=14.6700I = 14.6700

Why the other options are there

  • 29.3400 — kept a factor of two that cancels in the correct rearrangement.
  • 7.3350 — dropped that same factor in the other direction.
  • 16.1370 — rounded an intermediate value before the final step.

Reference: FE Handbook — Trapezoidal Rule

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