what is (l-r)^3
(l - r)3 = (l - r)(l - r)(l - r) = (l - r)[(l - r)(l - r)]
= (l - r)[1 - 2r + r²] FOILing (1 - r)(1 - r)
= 1 - 2r + r² - r + 2r² - r³] (multiplying out)
= 1 - 3r + 3r² - r³ (simplifying)