a)\(\left(x-2y-z+2t\right)\left(x-2y+z-2t\right)\)
\(=\left(x-2y\right)^2-\left(z-2t\right)^2\)
\(=x^2-4xy+4y^2-z^2+4zt-4t^2\)
b)\(ax^2+ay^2-bx^2-by^2+b-a\)
\(=a\left(x^2+y\right)-b\left(x^2+y^2\right)-\left(a-b\right)\)
\(=\left(a-b\right)\left(x^2+y^2\right)-\left(a-b\right)\)
\(=\left(a-b\right)\left(x^2+y^2-1\right)\)