1. (x^2 - xy) / (x^2-y^2) = x(x-y) / (x-y)*(x+y) = x/(x+y)
2. (a-z)(a+z)-a(a-1) = a^2+az-az-z^2-a^2+a = a-z^2
3. (a+b)(a-b)( a^(2)+b^(2) ) = (a^2-b^2)*(a^2+b^2) = a^4 - b^4
4. c(c-2)(c+2)-(c-1)(c^(2)+c+1)
(c-2)(c+2) = c^2-4
c(c-2)(c+2) = c(c^2-4) = c^3-4c
(c-1)(c^(2)+c+1) = c^3+c-c^2-c^2-c-1 = c^3-1
c(c-2)(c+2)-(c-1)(c^(2)+c+1) = c^3-4c - (c^3-1) = 1-4c