y=[(x-1)(x-4)][(x-3)(x-2)]+15 =(x^2-5x+4)(x^2-5x+6)+15 =[(x^2-5x+5)-1][(x^2-5x+5)+1]+15 =[x^2-5x+5]^2-1+15 x^2-5x+5=(x-5/2)^2-5/4>=-5/4; 所以[x^2-5x+5]^2>=0; y>=14;
