y的n-1次方乘以x2分之y的n次方=y^(n-1)*(y^n/x^2)=y^(2n-1)*x^-2
y的n次方分之x的n次方的相反數(shù)=1/(x^n/y^n)=y^n/x^n
原式等于：
y^(2n-1)*x^-2÷（y^n/x^n）^2
=y^(2n-1)*x^-2÷(y^2n/x^2n)
=y^(2n-1)*x^-2*x^2n/y^2n
=x^(2n-2)/y