x/(x+y)+x/(x-y)
=[x(x-y)+x(x+y)]/[(x+y)(x-y)]
=2x^2/(x^2-y^2)
由(2x)^2=16知x^2=4,由y=（-5）的平方正的平方根知y^2=25,代入上式
(2*4)/(4-25)=-8/21
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