59. one person has a fence, he uses the fence for a rectangle area, and the length is twice longer than the width. if he uses the fence for a square, the area will be 400 feet bigger. how long is the fence?
1. 16
2. 40
3.160
4. 1200
5. 1600
长x宽y
正方形z
x+y=2z
xy+400=z^2
求2(x+y)
(x-y)=400
x=2y
所以x=800,y=400
2400
所以答案应为 1600
X+Y=2Z
X=3Y
XY+400=Z^2
可以解出 Y=20 X=60 所以 2(X+Y)=160
define width= w, the total length of fence=x, then:
2w+3(2w)=x
(x/4)^2-3w^2=400
=> x=160
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