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问题: 谁帮我

计算:(1/5x+1/5y)*(-5x+5y)-y^2

解答:

计算:(1/5x+1/5y)*(-5x+5y)-y^2=
(1/5)(x+y)*(-5)(x-y)-y^2=
(1/5)*(-5)*(x+y)*(x-y)-y^2=
-(x^2-y^2)-y^2=
-x^2+y^2-y^2=
-x^2