Copie de Volume of a watermelon
Adjust the values of and until the purple function fits the top half of the watermelon well.
Then uncheck "Show watermelon" and check "Show rotated region".
If you're not sure what to do next, you may find it helpful to experiment with the applet here: http://higheredbcs.wiley.com/legacy/college/mccallum/0470131586/applets/ch8/hh_fig_8_16a.htm
What's the volume of the watermelon?