Originally Posted by VintageSailor

I'm stuck on these questions and need someone to show me how to do them. It's not that i was too lazy to do my homework, i've been off school for ages due to an illness and missed a lot so i don't know how to do these kind of questions yet. Hopefully if i see a demonstration i will be able to get the hang of it.

Q.1: An open box with a square base has the volume 256cm^3.
a) Taking x as the length of the base, show that the surface area of the box is x^2+1024/x

[[ This part i figured out ]]

b) Find the dimensions of the box that will minimise the surface area and calculate this area.

[[ The answer is 8 cm by 8cm by 4 cm, area= 192 cm .... how?]]

Q.2: The cost in millions, of pumping water at a hydroelectric station is related to the radius of the pipe carrying the water.
For a pipe the radius of r metres the cost is given by C = r + 9/r + 7
Find the radius which minimises the cost and calculate this cost.

[[ Answer is r = 3 metres, cost = £13 million ]]

Thank you (:

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EDIT:

Never mind!! After i spent ages waiting for someone to answer i ended up figuring it out myself!! Thanks for your help!! This forum is ****!