# Math Help - trigo problem no. 23

1. ## trigo problem no. 23

p198 q14 reex9
don't know how to do b ii and iii . thanks

2. Originally Posted by afeasfaerw23231233
p198 q14 reex9
don't know how to do b ii and iii . thanks
While I think about b ii ....

b iii. If $\cos A \cos B \cos C = 0$ then one of $\cos A$, $\cos B$ or $\cos C$ has to equal zero. Therefore one of A, B or C has to equal 90 degrees.

3. after thinking this question for thirty min. i finally know how to do b ii and iii. thanks anyway