(x-1)(x-3)(x-5)(x-7)-84=[(x-1)(x-7)][(x-3)(x-5)]-84=[(x²-8x)+7][(x²-8x)+15]-84=(x²-8x)²+22(x²-8x)+105-84=(x²-8x)²+22(x²-8x)+21=[(x²-8x)+1][(x²-8x)+21]...