用配方法,虽然比较麻烦
4x²-7x+2=0
x^2-7/4x=-1/2
x^2-7/4x+(7/8)^2=-1/2+(7/8)^2=17/64
(x-7/8)^2=17/64
x-7/8=±√17/8
x=7/8±√17/8