h: Ta có: \(x\left(x+2\right)\left(x+4\right)\left(x+6\right)+15\)
\(=\left(x^2+6x\right)\left(x^2+6x+8\right)+15\)
\(=\left(x^2+6x\right)^2+8\left(x^2+6x\right)+15\)
\(=\left(x^2+6x+3\right)\left(x^2+6x+5\right)\)
\(=\left(x+1\right)\left(x+5\right)\left(x^2+6x+3\right)\)