2x^2-6x+5=0
2(x^2-3x+5/2)=0
(x-3/2)^2-9/4+5/2=0
(x-3/2)^2-9/4+10/4=0
(x-3/2)^2+1/4=0
(x-3/2)^2=-1/4
∵平方数不可能为负
∴不论x为何值,该式不成立.