Đặt \(A=\left(x+3\right)^4+\left(x+5\right)^4-16\)
\(=\left\lbrack\left(x+4\right)-1\right\rbrack^4+\left\lbrack\left(x+4\right)+1\right\rbrack^4-16\)
Đặt b=x+4
=>\(A=\left(b-1\right)^4+\left(b+1\right)^4-16\)
\(=\left(b^2+2b+1\right)^2+\left(b^2-2b+1\right)^2-16\)
\(=\left(b^2+1\right)^2+4b\left(b^2+1\right)+4b^2+\left(b^2+1\right)^2-4b\left(b^2+1\right)+4b^2-16\)
\(=2\left(b^2+1\right)^2+8b^2-16\)
\(=2\left\lbrack\left(b^2+1\right)^2+4b^2-8\right\rbrack\)
\(=2\left\lbrack b^4+2b^2+1+4b^2-8\right\rbrack=2\left(b^4+6b^2-7\right)\)
\(=2\left(b^2+7\right)\left(b^2-1\right)=2\left(b^2+7\right)\left(b-1\right)\left(b+1\right)\)
\(=2\left\lbrack\left(x+4\right)^2+7\right\rbrack\left(x+4-1\right)\left(x+4+1\right)\)
\(=2\left(x^2+8x+23\right)\left(x+3\right)\left(x+5\right)\)
