Làm tính trừ: \(\frac{{12x}}{{x - 9}} - \frac{{x - 10}}{{81 - {x^2}}}\)

* Hướng dẫn giải

Ta có:

\(\begin{array}{l} \frac{{12x}}{{x - 9}} = \frac{{x - 10}}{{81 - {x^2}}} = \frac{{12x}}{{x - 9}} + \frac{{x - 10}}{{{x^2} - 81}}\\ = \frac{{12x}}{{x - 9}} + \frac{{x - 10}}{{(x + 9).(x - 9)}}\\ = \frac{{12x(x + 9) + x - 10}}{{(x + 9).(x - 9)}}\\ = \frac{{12{x^2} - 108x + x - 10}}{{(x + 9)(x - 9)}}\\ = \frac{{12{x^2} - 109x - 10}}{{(x + 9)(x - 9)}} \end{array}\)

Chọn đáp án C