化简:(x+y)(x-y)-2(4-y^2+1/2x^2) 赶紧回答吧,(x+y)(x-y)-2[4-y^2+(1/2)x^2] 是化简,不是计算!

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化简:(x+y)(x-y)-2(4-y^2+1/2x^2) 赶紧回答吧,
(x+y)(x-y)-2[4-y^2+(1/2)x^2] 是化简,不是计算!

x+y)(x-y)-2(4-y^2+1/2x^2) =x^2-y^2-8+2y^2-x^2=y^2-8