x^2 - 5x + 1 = 0
x^2+1=5x
两边除以x,(x可以得到不等于0)
x+1/x=5
3-10x+2x^2
=2(x^2-5x)+3
=2*(-1)+3
=1
x^2+1/x^2
=(x+1/x)^2-2
=5^2-2
=23