因为三角形ABC为锐角所以tanC＝tan[ ∏－（A＋B）]即tanC＝－（tanA＋tanB）÷（1－tanA×tanB）－tanC＝（tanA＋tanB）÷（1－tanA×tanB）－tanC＋tanA×tanB×tanC＝tanA＋tanB移项tanA×tanB×tanC＝tanA＋tanB...