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Question

Write and evaluate the definite integral that represents the area of the surface generated by revolving the curve on the indicated interval about the y-axis. y=x22+4y=\frac{x^{2}}{2}+4, 0x20 \leq x \leq 2

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Answered 9 months ago
Answered 9 months ago
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y=x22+4,0x2\color{#4257b2}{y=\frac{x^{2}}{2}+4, \quad 0 \leq x \leq 2}

Differentiate with respect to xx:

f(x)=ddx[x22+4]=2x2=x\begin{align*} f^{\prime}(x) &=\frac{d}{d x} [\frac{x^{2}}{2}+4] \\ &=\frac{2 x}{2} \\ &=x \end{align*}

Then the area of surface of revolution is:

S=2π02(x)(1+(x)2 dx)=2π02(x)1+x2 dx=2π[(x2+1)323]02=2π3(5321)=21.3\begin{align*} S &=2 \pi \int_{0}^{2}(x)\left(\sqrt{1+(x)^{2}} \ d x\right) \\ &=2 \pi \int_{0}^{2}(x) \sqrt{1+x^{2}} \ d x \\ &=2 \pi\left[\frac{\left(x^{2}+1\right)^{\frac{3}{2}}}{3}\right]_{0}^{2} \\ &=\frac{2 \pi}{3}\left(5^{\frac{3}{2}}-1\right) \\ &=21.3 \end{align*}

S=21.3\color{#c34632}{S=21.3}

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