x^2-2x+5=0
△=b^2-4ac
=2^2-4*1*5
=4-20
=-16<0
方程无解
x^2-2√7x+7=0
x^2-2√4x+(√7)^2=0
(x-√7)^2=0
x=√7
2x^2-x-6=0
(2x+3)(x-2)=0
x=-3/2或x=2
x^2-7x+2=0
x^2-7x+49/4-49/4+2=0
(x-7/2)^2-41/4=0
(x-7/2-√41/2)(x-7/2+√41/2)=0
x=7/2+√41/2或x=7/2-√41/2