y=[(x-1)(x-4)][(x-2)(x-3)]+15 =[(x^2-5x)+4][(x^2-5x)+6]+15 =(x^2-5x)^2+10(x^2-5x)+24+15 =(x^2-5x)^2+10(x^2-5x)+39 x^2-5x=(x-5/2)^2-25/4>=-25/4 令a=x^2-5x 则a>=-25/4 y=a^2+10a+39 =(a+5)^2+14 a>=-25/4 所以a=-5时,y最小值=14