B2-18b-c2+81 b² -18b + 81 - c² (b - 9)² - c²
---------------------------- = -------------------------------- = ------------------------ =
b2+2bc+c2-9b-9c b² + 2bc + c² - 9(b +c) (b + c)² - 9(b + c)
(b - 9 - c)(b - 9 + c) b - c - 9
= ----------------------------- = -------------------
(b + c)(b + c - 9) b + c