MHB Find Vol Bound by $x_2 =\frac{y+1}{2}$ & $x_1=y^2$

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The discussion focuses on finding the volume bound by the equations x2 = (y + 1)/2 and x1 = y^2. The integral used for calculation is ∫ from -1/2 to 1 of (-y^2 + y/2 + 1/2) dx. The calculations lead to the result of 9/16 for the volume. Participants emphasize the importance of careful arithmetic in solving such integrals, confirming that the final answer is correct. The conversation highlights the challenges and satisfaction in solving complex mathematical problems.
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Find volume bound by $x_2 =\frac{y+1} {2} $ and $x_1=y^2$
$\int_{-{\frac{1}{2}}}^{1} (-{y}^{2 }+\frac{y}{2}+\frac{1}{2}) \,dx$
$
\begin{array}{l}
{{\left[{\frac{{y}^{2}}{2}\mathrm{{+}}\frac{y}{4}\mathrm{{-}}\frac{{y}^{3}}{3}}\right]}_{\mathrm{{-}}{1}{\mathrm{/}}{2}}^{1}}\\
{\left[{\frac{\frac{1}{4}}{2}\mathrm{{-}}\frac{\frac{1}{2}}{4}\mathrm{{+}}\frac{\frac{1}{8}}{3}}\right]\mathrm{{-}}\left[{\frac{1}{4}\mathrm{{+}}\frac{1}{2}\mathrm{{-}}\frac{1}{3}}\right]}\\
{\left[{\frac{3}{\mathrm{48}}\mathrm{{-}}\frac{\mathrm{12}}{\mathrm{48}}\mathrm{{+}}\frac{2}{\mathrm{48}}}\right]\mathrm{{-}}\left[{\frac{\mathrm{12}}{\mathrm{48}}\mathrm{{+}}\frac{\mathrm{24}}
{\mathrm{48}}\mathrm{{-}}\frac{\mathrm{16}}{\mathrm{48}}}\right]}
\end{array}$
$$
\frac{5}{12 }+\frac{7}{48 }=\frac{9}{16}
$$
Thot I would try mathmajic but better to stay here😁
 
Last edited:
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karush said:
Find volume bound by $x_2 =\frac{y+1} {2} $ and $x_1=y^2$
$\int_{-{\frac{1}{2}}}^{1} (-{y}^{2 }+\frac{y}{2}+\frac{1}{2}) \,dx$
$
\begin{array}{l}
{{\left[{\frac{{y}^{2}}{2}\mathrm{{+}}\frac{y}{4}\mathrm{{-}}\frac{{y}^{3}}{3}}\right]}_{\mathrm{{-}}{1}{\mathrm{/}}{2}}^{1}}\\
{\left[{\frac{\frac{1}{4}}{2}\mathrm{{-}}\frac{\frac{1}{2}}{4}\mathrm{{+}}\frac{\frac{1}{8}}{3}}\right]\mathrm{{-}}\left[{\frac{1}{4}\mathrm{{+}}\frac{1}{2}\mathrm{{-}}\frac{1}{3}}\right]}\\
{\left[{\frac{3}{\mathrm{48}}\mathrm{{-}}\frac{\mathrm{12}}{\mathrm{48}}\mathrm{{+}}\frac{2}{\mathrm{48}}}\right]\mathrm{{-}}\left[{\frac{\mathrm{12}}{\mathrm{48}}\mathrm{{+}}\frac{\mathrm{24}}
{\mathrm{48}}\mathrm{{-}}\frac{\mathrm{16}}{\mathrm{48}}}\right]}
\end{array}$
$$
\frac{5}{12 }+\frac{7}{48 }=\frac{9}{16}
$$
Thot I would try mathmajic but better to stay here😁

9/16 is correct. Well done.
 
It's the arithmetic that kills on these integrals
 
There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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