Erfana Beevee Binte Hassan Shariff's answer to Ryan's Secondary 4 A Maths Singapore question.

done {{ upvoteCount }} Upvotes
clear {{ downvoteCount * -1 }} Downvotes
Erfana Beevee Binte Hassan Shariff
Erfana Beevee Binte Hassan Shariff's answer
275 answers (Tutor Details)
Part 2/2 of solution.
First, express the total surface area of figure in terms of h and r and equate to 24pi.
Then express h in terms of r since the volume to be shown is in terms of r.
Then express volume of figure in terms of h and r and sub the expression of h into it.
Then differentiate w.r.t r. Equate dV/dr=0 and you will get 2 values.
In fact, r cannot take negative value. So the negative value can be rejected. So the volume is a maximum when r takes the positive value.
Hope this helps :)