\(\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15\)
\(=x^4+9x^3+23x^2+15x+7x^3+63x^2+161x+105+15x^4\\ +135x^3+345x^2+225x+105x^3+945x^2+2415x+1575+15\)
\(=16x^4+256x^3+1376x^2+2816x+1695\)
\(=16x^3\left(x+16\right)+32x\left(43x+88\right)+1695\)
......
Hình như đề phải là (x+1)(x+3)(x+5)(x+7)+15
\(\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15=\left[\left(x+1\right)\left(x+7\right)\right]\left[\left(x+3\right)\left(x+5\right)\right]+15\)
\(=\left(x^2+8x+7\right)\left(x^2+8x+15\right)+15\)
Đặt \(x^2+8x=a\Rightarrow\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15\)
\(=\left(a+7\right)\left(a+15\right)+15=a^2+22a+105+15=a^2+22a+120\)
\(=\left(a^2+22a+121\right)-1=\left(a+11\right)^2-1^2=\left(a+11-1\right)\left(a+11+1\right)\)
\(=\left(a+10\right)\left(a+12\right)=\left(x^2+8+10\right)\left(x^2+8+12\right)\)
\(=\left(x^2+18\right)\left(x^2+20\right)\)