4x^2 - 12x +15 = 4(x^2 - 3x + 9/4 ) + 6 + 9= 4(x - 3/2 )^2 +15
4x^2-12x+15=4(x-3/2)^2+6
4x^2-12x+15=4(3/2)^2+6化简后得：4x^2-12x+15=15所以配方得4(x-3/2)^2=9
