此题用的是完全平方公式
（x+y)^2=x^2+2xy+y^2
(x-y)^2=x^2-2xy+y^2
所以（x+y)^2-(x-y)^2=4xy
所以原式等于4xy/2xy=2
[(x+y)²-(x-y)²]÷2xy
=[(x²+2xy+y²)-(x²-2xy+y²)]÷2xy
=4xy÷2xy
=2
解：
[(x+y)²-(x-y)²]÷2xy
=[(x+y-(x-y))*(x+y+(x-y))]÷2xy
=2y*2x÷2xy
=2
[(x+y)²-(x-y)²]÷2xy
=(4xy)÷2xy
=2
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