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4. (b) show that the surface area A m^2 of the box is given by A=x^2 + 80/x.

* I got h in terms of x for 4(a) that was easy it is h=20/x^2 and it is right from the answer. just how do i find surface area, because i know it is the addition of all the area of the sides but i got it wrong.

5.AN open cylintrical can of radius r cm and height hcm is to be made out of 25 pi(cm)^2 of sheet tin.

(a) express the height hcm in terms of r.

I got 25pi - pi(r)^2 /2pi(r) = h

the back of the book got h = 25-r^2 / 2r someone guide me thorugh on the steps on how to get the answer.

(b) express volume in terms of r.

i know volume = area * h. and to do that i need pi(r)^2 * h from previous answer. so someone show me how to do that? because i got v=25pi(r)-pi(r)^3 2 i know that is the wrong answer because the back of the book got v=(1/2)r(25-r^2) someone show me how to get it thanks.

9. A skeleton cylinder is made of two identical circular pieces of wire joined by two identical striaght pieces the legnth of the wire used is 32cm and the radius of the cylinder is r cm.

(a) express the height hcm in terms of r

* well i am giving a radium and the side lengths, but i got condused and i did 32 + 2pi(r)^2 + 2h. I am really lost but the answer is

h=2(8-pi(r))

(b) express the volume vcm^3 of the cylinder in therms of r. state restrictions

Ok i can only do that if i have h, well from if someone tells me how to get the previous answer, i just dop v = a * h and rea is just (pi)r^2 * 2??? because there are 2 cylinder heads. the restrictions is easy just to see the numbers which are in the positive maxium etc.

i tried my best someone please answer how to get the h= thanks.

10. A box has a rectangular base the diagnoal of which is 6m

(a) if the width of the base is wm, express length, lm of the base in terms of w

well i know that i have to get it in the form of l=

and, i suppose 6m is the side diagnol. i got ripped and did l^2 + 36 + w^2 i'm really lost someone please help me on how they got l=sqr(36-w^2)

(b) if the height of the box is equal to the length of the base, express the volume vm^3 in terms of w. state any restrictions on w.

* if height is equal to length, then volme = area * h where area is (l*w) and length from above * width but i dont know how to solve this. someone help me?

* SORRY TO BOTHER YOU PEOPLE BUT I AM REALLY STUCK PLEASE SHOW ME WHAT TO DO BECAUSE I KNOW I DID IT WRONG THANKS!