题目内容：

the length of a rectangle is twice its width.
the length of a rectangle is twice its width.If the width of the rectangle is changing at 2in/s,how fast is the area of the rectangle changing when the area is 32 in^2?

优质解答

area = width * length = 2width^2
d area/dt = 4width dwidth/dt
when area=32in^2,width=4
so d area/dt = 4*4*2 = 32 in/s