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hey guys got this maximum volume question so bloody hard !!! i got most of the steps right but got stuck on the last section (V)

1.

A closed box has a surface area of 18 square metres. let the dimensions of the three sides x,y,z and let volume be V

(i) write the volume V in terms of x,y,z

aight i got this right:

V=xyz

(ii) use result that surface area is 18 s metrs, find z in terms of x and y

1 got

Z = (9-xy)/(x+y)

(iii)

Find V in terms of x and y with results from (i) and (ii)

V = (9xy - x^2y^2) / (x+y)

(iv) using reults in (iii) find dv/dx and dv/dy where "d" represents the "day symbol"

i got

dv/dx = (9y^2-x^2y-2xy^3) / ((x+y)^2)

dv/dy = (9x^2-x^2y-2x^3y) / ((x+y)^2)

(v) for values of x,y,z to get max volume, i tried to let dv/dx and dv/dy = 0 no luck at all please help!!! thanks

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# Maximum Volume Hard

**Physics Forums | Science Articles, Homework Help, Discussion**