final answers whited out, simply insert tex tags to view:

the answer for a) is

b)

i understand how you derived at

but how come the

becomes

despite the fact that the answers do align when

is inserted. Also the reduction to the formula

i quite dont get..

and the miscalculation at the end of your working is due to leaving out the multiplication of

which is

wait i think i get it, is it like this, instead of going the long way aroud, its supposed to be:

=

which then equals ;

anyways thank you for the clarification of the situation.

As for the rectangle question, try differentiating this instead.

thank you for the correction, forgot to half the 90 degree in to 45/45, therefore making it half radian.