\(D=\left(x-2\right)\left(x-4\right)\left(x-6\right)\left(x-8\right)+15\)
\(D=\left[\left(x-2\right)\left(x-8\right)\right]\left[\left(x-4\right)\left(x-6\right)\right]+15\)
\(D=\left(x^2-10x+16\right)\left(x^2-10x+24\right)+15\)
Đặt x2 - 10x + 20 = a, ta được
\(D=\left(a-4\right)\left(a+4\right)+15\)
\(D=a^2-16+15\)
\(D=a^2-1\)
\(D=\left(a-1\right)\left(a+1\right)\)
\(D=\left(x^2-10x+20-1\right)\left(x^2-10x+20+1\right)\)
\(D=\left(x^2-10x+19\right)\left(x^2-10x+21\right)\)
\(D=\left(x^2-10x+19\right)\left(x^2-3x-7x+21\right)\)
\(D=\left(x^2-10x+19\right)\left[x\left(x-3\right)-7\left(x-3\right)\right]\)
\(D=\left(x^2-10x+19\right)\left(x-3\right)\left(x-7\right)\)
