先化简再求值2x/(x²-y²)+x/(y²-x²)+y/[(x-y)(x+y)
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先化简再求值2x/(x²-y²)+x/(y²-x²)+y/[(x-y)(x+y)],其中x=-3,y=4
原式 = 2 x / (x + y)(x - y)- x /(x + y)(x - y)+ y / (x + y)(x - y)
= (2 x - x + y)/(x + y)(x - y)
= (x + y)/ (x + y)(x - y)
= 1 / (x - y)
当 x = - 3 ,y = 4 时,则原式 = 1 / (- 3 - 4)
= 1 / (- 7)
= - 1 / 7
= (2 x - x + y)/(x + y)(x - y)
= (x + y)/ (x + y)(x - y)
= 1 / (x - y)
当 x = - 3 ,y = 4 时,则原式 = 1 / (- 3 - 4)
= 1 / (- 7)
= - 1 / 7
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