(x的四次方+y的四次方)÷(xy)的平方
=(x的四次方+y的四次方-2x²y²+2x²y²)÷(xy)的平方
=[(x²-y²)²+2x²y²]÷(xy)的平方
=[(4xy)²+2x²y²]÷(x²y²)
=18x²y²÷(x²y²)
=18