Question

tan(A+B) = (tanA+tanB) / (1-tanA * tanB) = -1从而得到:tanA+tanB = tanA * tanB -1

tan(A+B) = (tanA+tanB) / (1-tanA * tanB) = -1
从而得到:
tanA+tanB = tanA * tanB -1

不明白为什么要-1

Answer 1

(tanA+tanB) / (1-tanA * tanB) = -1

两边同乘以 (1-tanA * tanB)，等式两边就为
(tanA+tanB)= - (1-tanA * tanB)，“-“(1-tanA * tanB)注意这个外面有一个负号打开括号后
(tanA+tanB)= - (1-tanA * tanB)=tanA * tanB-1

Answer 2

tan(A+B) = (tanA+tanB) / (1-tanA * tanB) = -1

tanA+tanB = (tanA * tanB+1)*(-1)

= tanA * tanB -1
-1是前面1和后面（-1）相乘得来的。

Answer 3

(tanA tanB) / (1-tanA * tanB) = -1
［ (tanA tanB) / (1-tanA * tanB) ］(1-tanA * tanB) = -1 (1-tanA * tanB)
(tanA tanB) = -1＋tanA * tanB
tanA tanB ＝tanA * tanB -1