利用作差比较法
4-(4-x^2)(x-3)(x+1)
=4+(x²-4)(x²-2x-3)
=4+(x²-4)[(x-1)²-4]
=x²(x-1)²-4[x²+(x-1)²]+20
=x²(x-1)²-4(2x²-2x+1)+20
=x²(x-1)²-8x(x-1)+16
=[x(x-1)+4]²
≥0
∴ 4≥ (4-x^2)(x-3)(x+1)
即无论x取何实数,代数式(4-x^2)(x-3)(x+1)的值都不大于4
利用作差比较法
4-(4-x^2)(x-3)(x+1)
=4+(x²-4)(x²-2x-3)
=4+(x²-4)[(x-1)²-4]
=x²(x-1)²-4[x²+(x-1)²]+20
=x²(x-1)²-4(2x²-2x+1)+20
=x²(x-1)²-8x(x-1)+16
=[x(x-1)+4]²
≥0
∴ 4≥ (4-x^2)(x-3)(x+1)
即无论x取何实数,代数式(4-x^2)(x-3)(x+1)的值都不大于4