Help !!! integration/differentiation

(Doh)

Hey, thanks for looking.

I have this q/

A cylindrical tank, open at the top,has height hm and raduis rm.Its capacity is 1m^3.

i) show that h=1/pie r^2. Which i did..1m^3=pie r^2 multiply by h, 1m^3-pie r^2=h, 1mpier^2=h, then finally 1/pier^2=h. Is this the correct workin?

ii) Also, its total internal surface area is Sm^2. show that S=2/r +pier^2. Which iv done as,h multiplied by 2pier=2pie2h, pier^2+2pierh =pier^2+2pier.. but not sure where from here..

iii) finally...Determine the value of r which makes the surface area S as small as possible..For this i need to make dy/dx and the 2nd diff, or integration..im not sure, either way could you show me as not sure how to do it with fractions..

thank you