1) ta có : \(x^2-4-6xy+9y^2=\left(x-3y\right)^2-2^2=\left(x-3y+2\right)\left(x-3y-2\right)\)
2) ta có : \(9a^2+6ab+b^2-49x^2=\left(3a+b\right)^2-\left(7x\right)^2=\left(3a+b-7x\right)\left(3a+b+7x\right)\)
3) ta có : \(b^4+4a^4=b^4+4a^2b^2+4a^4-4a^2b^2=\left(b^2+2a^2\right)^2-\left(2ab\right)^2\)
\(=\left(b^2+2a^2-2ab\right)\left(b^2+2a^2+2ab\right)\)