\(4x^4+81\)
\(=4x^4+81+36x^2-36x^2\)
\(=\left(2x^2\right)^2+2.2x^2.9+9^2-36x^2\)
\(=\left(2x^2+9\right)^2-\left(6x\right)^2\)
\(=\left(2x^2+9-6x\right)\left(2x^2+9+6x\right)\)
Ta có: \(4x^4+81=\left(2x^2\right)^2+2.2x^2.9+9^2-36x^2\)
\(=\left(2x^2+9\right)^2-\left(6x\right)^2\)
\(=\left(2x^2-6x+9\right)\left(2x^2+6x+9\right)\)
Ta có :
\(4x^4+81=4x^4+81+36x^2-36x^2\)
\(=\left(4x^4+36x^2+81\right)-36x^2\)
\(=\left[\left(2x\right)^2+9\right]^2-\left(6x\right)^2\)
\(=\left(2x^2+9-6x\right)\left(2x^2+9+6x\right)\)
4x4 + 81
= ( 2x2)2 + 2.9.2x2 + 92 - 36x2
= ( 2x2 + 9)2 - ( 6x)2
= ( 2x2 + 9 - 6x )( 2x2 + 9 + 6x)
