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Can someone tell me why the answers highlighted in red ( 12 and pi/2 ) are wrong and how can I find the correct answers? The rest are correct enter image description here

Thank you!

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  • $\begingroup$ Just draft the projection on the $xy$ plane and you'll immediately see what's wrong. Consider that $x^2+y^2=8y$ can be written as $x^2+(y-4)^2=16$. $\endgroup$
    – Yes
    Mar 9 '17 at 2:45
  • $\begingroup$ I'm sorry, I don't get it $\endgroup$ Mar 9 '17 at 2:58
  • $\begingroup$ Since the projection on the $xy$ plane is a circle with a diameter of 8, it can be easily shown that $0\leq y\leq 8$. $\endgroup$
    – Yes
    Mar 9 '17 at 3:09
  • $\begingroup$ oh yeah you're right. what about the angle theta? $\endgroup$ Mar 9 '17 at 3:12
  • $\begingroup$ To cover the whole domain you need $0\leq\theta\leq \pi$. $\endgroup$
    – Yes
    Mar 9 '17 at 3:23

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