4x-3x^2-8
=-3x^2+4x-8
=-3(x-2/3)^2-20/3
当x=2/3时,有最大值-20/3=-3(x-2/3)^2-20/3是怎么得来的？-3x^2+4x-8=-3[x^2-2×(2/3)x+(2/3)^2]-8+(-3)(2/3)^2=-3(x-2/3)^2-20/3