已知1/x+1/y=4,求(x+y)²+4x²y²/x²y+xy²的值
[(x+y)²+4x²y²]/(x²y+xy²)
=[(1/x+1/y)²+4]/(1/x+1/y)（分子分母同时÷x²y²）
=[4²+4]/4
=20/4
=5