\(=\left(x+a\right)\left(x+4a\right)\left(x+2a\right)\left(x+3a\right)+a^4\)
\(=\left(x^2+5ax+4a^2\right)\left(x^2+5ax+6a^2\right)+a^4\)
đặt \(x^2+5ax+5a^2=b\)
\(=\left(b-a^2\right)+\left(b+a^2\right)+a^4\)
\(=b^2-a^4+a^4=b^2=\left(x^2+5ax+5a^2\right)^2\)