MHB How to use change of variables technique here?

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To find the volume inside both the sphere defined by $x^2 + y^2 + z^2 = 1$ and the cone given by $z = \sqrt{x^2 + y^2}$, the change of variables technique is applied using cylindrical coordinates. In this method, z remains unchanged while x and y are substituted with polar coordinates r and θ. The boundary equations are transformed to $r^2 + z^2 = 1$ and $z = r$. The volume is calculated using the integral $\int_{z=0}^1 \int_{\theta=0}^{2\pi} \int_{r=0}^1 r \, dr \, d\theta \, dz$, leading to a solution for the volume. The discussion concludes with the problem being marked as 'SOLVED'.
WMDhamnekar
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Find the volume V inside both the sphere $x^2 + y^2 + z^2 =1$ and cone $z = \sqrt{x^2 + y^2}$

My attempt: I graphed the cone inside the sphere as follows. But I don't understand how to use the change of variables technique here to find the required volume. My answer without using integrals is volume of the cone + volume of the spherical cap = $\frac{\pi}{3} \times (\frac12) \times \frac{1}{\sqrt{2}} + \frac{ \pi}{3} *(1-\frac{1}{\sqrt{2}})^2 *(\frac{3}{\sqrt{2}}-(1-\frac{1}{\sqrt{2}}))= 0.497286611528$

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I would change to "cylindrical coordinates" in which z is kept and x and y are replaced by polar coordinates r and $\theta$. The boundary equations become $x^2+ y^2+ z^2= r^2+ z^2= 1$ and $z= r$. The "differential of volume" is $r drd\theta dz$. r goes from 0 to 1, $\theta$ goes from 0 to $2\pi$, and z goes from 0 to 1. The volume is given by $\int_{z= 0}^1\int_{\theta= 0}^{2\pi}\int_{r= 0}^1 rdrd\theta dz$
 
HallsofIvy said:
I would change to "cylindrical coordinates" in which z is kept and x and y are replaced by polar coordinates r and $\theta$. The boundary equations become $x^2+ y^2+ z^2= r^2+ z^2= 1$ and $z= r$. The "differential of volume" is $r drd\theta dz$. r goes from 0 to 1, $\theta$ goes from 0 to $2\pi$, and z goes from 0 to 1. The volume is given by $\int_{z= 0}^1\int_{\theta= 0}^{2\pi}\int_{r= 0}^1 rdrd\theta dz$
This question is 'SOLVED'
 
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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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