Сократить дробь а^2-16а+63/а^2-81

[latex] \frac{a^2-16a+63}{a^2-81} = \frac{a^2-7a+9a+63}{(a-9)(a+9)} = \frac{a(a-7)-9(a-7)}{(a-9)(a+9)} = \frac{(a-9)(a-7)}{(a-9)(a+9)} = \frac{a-7}{a+9} [/latex]