Question

已知xy-y^2=5,且x,y都是正整数,求5分之2乘以（xy^3-y^4)的值.

Answer 1

xy-y^2=y(x-y)=5

因为x和y都是正整数,所以 y=1 y=5

2/5(xy^3-y^4)=2/5y^2(xy-y^2)=2y^2

当y=1时,原式=2

当y=5时,原式=50