[問題]GWD 10-06

[lunar916]

GWD 10-6
If x and y are positive, what is the value of x - y?
1) (x^2 - y^2) / (x + y) = 4
2) x + y =7


ans: A

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不會算...
可以教一下嘛?

[yuying]

1)
x^2 - y^2 = (X+Y) (X-Y)
(X+Y) (X-Y) / (X+Y) = X-Y =4 可知X-Y
因為X,Y為正數, X+Y不為零, 上下同時約掉

2) X+Y =7 , 不可知X-Y