你给的条件好像不够ax^2+bx+c=0有两个相等的实根 => b^2-4ac=0若c=1，则b^2=4aab²/（a-2）²-（b²-4）=a*4a/(a-2)^2-(4a-4)=4a^2/(a-2)^2-4(a-1)=4a^2-4(a-1)(a-2)^2/(a-2)^2=(-a^3+9a^2-8a+4)/(a-2)^2
