2x^2-7x+2=0
显然x不等于0
所以两边可以除以x
2x-7+2x^-1=0
x+x^-1=7/2
两边平方
x^2+2x*x^-1+x^-2=49/4
x^2+2+x^-2=49/4
x^2+x^-2=41/4