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