a: \(=\left(x-1\right)\left(x+1\right)\left(x^2+1\right)\)
c: \(=\left(x-a-x-a\right)\left(x-a+x+a\right)\left(x^2+2ax+a^2+x^2-2ax+a^2\right)\)
\(=-2a\cdot2x\cdot\left(2x^2+2a^2\right)\)
\(=-8ax\left(x^2+a^2\right)\)
d: \(=a^4-\left(2ab-b^2\right)^2\)
\(=\left(a^2-2ab+b^2\right)\left(a^2+2ab-b^2\right)\)
\(=\left(a-b\right)^2\left(a^2+2ab-b^2\right)\)