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Evaluating a definite integral using the power rule antiderivative

Evaluate the definite integral: ∫18x1/3 dx\int_1^8 x^{1/3}\, dx

Find an antiderivative using the power rule, then apply the Fundamental Theorem of Calculus (Part 2) to compute the exact value.

Hint

First find an antiderivative F(x)F(x) of x1/3x^{1/3} using the power rule for integration: ∫xn dx=xn+1n+1+C\int x^n\,dx = \frac{x^{n+1}}{n+1} + C. Then compute F(8)−F(1)F(8) - F(1), using the fact that 84/3=(81/3)4=248^{4/3} = (8^{1/3})^4 = 2^4.

Step-by-step solution

Using the power rule for antiderivatives, ∫x1/3 dx=x4/34/3+C=34x4/3+C\int x^{1/3}\,dx = \frac{x^{4/3}}{4/3} + C = \frac{3}{4}x^{4/3} + C. Since we only need one antiderivative for FTC Part 2, take F(x)=34x4/3F(x) = \frac{3}{4}x^{4/3}.

By FTC Part 2, ∫18x1/3 dx=F(8)−F(1)=34(8)4/3−34(1)4/3\int_1^8 x^{1/3}\,dx = F(8) - F(1) = \frac{3}{4}(8)^{4/3} - \frac{3}{4}(1)^{4/3}. Since 81/3=28^{1/3}=2, we get 84/3=24=168^{4/3} = 2^4 = 16, so the expression becomes 34(16)−34(1)=12−34=454\frac{3}{4}(16) - \frac{3}{4}(1) = 12 - \frac{3}{4} = \frac{45}{4}.

Identifying which value to subtract in FTC Part 2

Hint
Step-by-step solution

The correct answer is lower limit. By FTC Part 2, the integral equals the antiderivative evaluated at the upper limit minus the antiderivative evaluated at the lower limit. In the worked example, this meant computing F of eight minus F of one, so the value at the lower limit, one, was the quantity being subtracted.

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