p198 q14 reex9
don't know how to do b ii and iii . thanks
While I think about b ii ....
b iii. If $\displaystyle \cos A \cos B \cos C = 0$ then one of $\displaystyle \cos A$, $\displaystyle \cos B$ or $\displaystyle \cos C$ has to equal zero. Therefore one of A, B or C has to equal 90 degrees.