a) \(a^5+a^3-a^2-1\)
\(=a^5+a^4+a^3+a^3+a^2+a-a^4-a^3-a^2-a^2-a-1\)
\(=a^3\left(a^2+a+1\right)+a\left(a^2+a+1\right)-a^2\left(a^2+a+1\right)-\left(a^2+a+1\right)\)
\(=\left(a^3+a-a^2-1\right)\left(a^2+a+1\right)\)
\(=\left[\left(a^3-1\right)-a\left(a-1\right)\right]\left(a^2+a+1\right)\)
\(=\left[\left(a-1\right)\left(a^2+a+1\right)-a\left(a-1\right)\right]\left(a^2+a+1\right)\)
\(=\left(a-1\right)\left(a^2+a+1-a\right)\left(a^2+a+1\right)\)
\(=\left(a-1\right)\left(a^2+1\right)\left(a^2+a+1\right)\)
b) \(27a^2b^2-18ab+3\)
\(=3\left(9a^2b^2-6ab+1\right)\)
\(=3\left(3ab-1\right)^2\)
c) \(4-x^2-2xy-y^2\)
\(=4-\left(x+y\right)^2\)
\(=\left(2-x-y\right)\left(2+x+y\right)\)
