By using the substitution u=1+t3u = 1 + t^3, find the exact value of ∫02t5(1+t3)3 dt\int_{0}^{2} \frac{t^5}{\left(1 + t^3\right)^3} \text{ d}t without using a calculator.

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Worked solution

3

1. Find the derivative of the substitution to relate $du$ and $dt$.

u=1+t3u = 1 + t^3

dudt=3t2\frac{\text{d}u}{\text{d}t} = 3t^2

t2 dt=13 dut^2 \text{ d}t = \frac{1}{3} \text{ d}u

2. Determine the new limits of integration in terms of $u$.

When t=0t = 0, u=1+03=1u = 1 + 0^3 = 1.

When t=2t = 2, u=1+23=9u = 1 + 2^3 = 9.

3. Rewrite the integrand entirely in terms of $u$.

Rewrite t5t^5 as t3⋅t2t^3 \cdot t^2:

t3=u−1t^3 = u - 1

∫02t5(1+t3)3 dt=∫02t3(1+t3)3⋅t2 dt\int_{0}^{2} \frac{t^5}{\left(1 + t^3\right)^3} \text{ d}t = \int_{0}^{2} \frac{t^3}{\left(1 + t^3\right)^3} \cdot t^2 \text{ d}t

=∫19u−1u3(13) du= \int_{1}^{9} \frac{u - 1}{u^3} \left(\frac{1}{3}\right) \text{ d}u

=13∫19(uu3−1u3) du= \frac{1}{3} \int_{1}^{9} \left( \frac{u}{u^3} - \frac{1}{u^3} \right) \text{ d}u

=13∫19(u−2−u−3) du= \frac{1}{3} \int_{1}^{9} \left( u^{-2} - u^{-3} \right) \text{ d}u

4. Integrate the function with respect to $u$.

=13[u−1−1−u−2−2]19= \frac{1}{3} \left[ \frac{u^{-1}}{-1} - \frac{u^{-2}}{-2} \right]_{1}^{9}

=13[−1u+12u2]19= \frac{1}{3} \left[ -\frac{1}{u} + \frac{1}{2u^2} \right]_{1}^{9}

5. Evaluate the definite integral using the upper and lower limits.

=13[(−19+12(92))−(−11+12(12))]= \frac{1}{3} \left[ \left( -\frac{1}{9} + \frac{1}{2(9^2)} \right) - \left( -\frac{1}{1} + \frac{1}{2(1^2)} \right) \right]

=13[(−18162+1162)−(−1+12)]= \frac{1}{3} \left[ \left( -\frac{18}{162} + \frac{1}{162} \right) - \left( -1 + \frac{1}{2} \right) \right]

=13[−17162−(−12)]= \frac{1}{3} \left[ -\frac{17}{162} - \left( -\frac{1}{2} \right) \right]

=13[−17162+81162]= \frac{1}{3} \left[ -\frac{17}{162} + \frac{81}{162} \right]

=13[64162]=13[3281]=32243= \frac{1}{3} \left[ \frac{64}{162} \right] = \frac{1}{3} \left[ \frac{32}{81} \right] = \frac{32}{243}

Answer

32243\frac{32}{243}

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