\(a,x^2-y^2+10yz-25z^2\\ =x^2-\left(y^2-10yz+25z^2\right)\\ =x^2-\left(y-5z\right)^2\\ =\left(x-y+5z\right)\left(x+y-5z\right)\\ ---\\ b,a^2-x^2+4ab+4xy+b^2-4y^2\\ =\left(a^2+2ab+b^2\right)-\left(x^2-2xy+4y^2\right)+2ab\\ =\left(a+b\right)^2-\left(x-2y\right)^2-2ab\)