Could you please check my working and help me complete..thank you

I have a q..bin with volume pie/8m^3 made from metal, to keep costs low it should have the min possible surface area, Am^2.

* using pie/8m^3 write an expression for height t in terms of radius r..

Which iv done pie/8m=pier^2Xh, then , pie/8 - pier^2=h, then, pie/8pie r^2=h, then, 1/8r^2=h, is this write?

then show this expressen as A=pier^2+pie/4r

area of base= pier^2

curved area side hX2pier=2pie r h

total area pier^2+2pie r h

pie r^2+2 pie r 1/8r^2

and finally A=pie r ^2+ pie/4r is this right?

then it says to find dA/r and d^2A/dr^2

which i have done 2pier-pie/4r^2 then 2pie+pie/2r^3

But finally i need to find the corresponding minimum values..but im not sure how too so this....

can you tell me how please?

Thank you