Chapter 14.4, Problem 6E

Solution

To find: The measures of angles $\angle B$ and $\angle C$ of a triangle ABC .

The measurements of angles $\angle B$ and $\angle C$ are $m\angle B=16.5°$ and $m\angle C={8.5}^{\text{o}}$ .

Given data: The measure of angle $m\angle A=155°$ , $a=12.5$ , and $b=8.4$ .

Method/Formula used:

The Sine Law of triangle ABC shown in Fig. 1 is

  

  $\frac{\sin A}{a}=\frac{\sin B}{b}=\frac{\sin C}{c}$ … (1)

The sum of angles of triangle $m\angle A+m\angle B+m\angle C={180}^{\text{o}}$ … (2)

Calculations:

The measure of angle $m\angle A={155}^{\text{o}}$ , $a=12.5$ , and $b=8.4$ .

Substitute ${155}^{\text{o}}$ for $\angle A$ , $12.5$ for ‘ a’ and $8.4$ for b in the result (1),

  $\frac{\sin 155°}{1.5}=\frac{\sin \angle B}{8.4}=\frac{\sin \angle C}{c}$

Taking first two ratios,

  $\frac{\sin 155°}{12.5}=\frac{\sin \angle B}{8.4}$

  $\begin{array}{c}\sin \angle B=\frac{8.4\sin 155°}{12.5}\\ \angle B={\sin }^{-1}\left(\frac{8.4\sin 155°}{12.5}\right)\\ \angle B={\sin }^{-1}\left(0.2840\right)\\ =16.5°\end{array}$

Since sine is positive in second quadrant so another value of $\angle B$ is $180°-16.5°$ or $163.5°$

Thus, in triangle ABC , $m\angle A={155}^{\text{o}}$ , $m\angle B={16.5}^{\text{o}}$ , therefore, from result (2),

  $m\angle C={180}^{\text{o}}-\left(m\angle A+m\angle B\right)$

Substitute ${155}^{\text{o}}$ for $m\angle A$ and ${16.5}^{\text{o}}$ for $m\angle B$

  $\begin{array}{c}m\angle C={180}^{\text{o}}-\left(155°+16.5°\right)\\ ={8.5}^{\text{o}}\end{array}$

Since in a triangle only one angle can be obtuse, therefore, $\angle B\ne 163.5°$ .

Therefore, the measurements of angles $\angle B$ and $\angle C$ are $m\angle B=16.5°$ and $m\angle C={8.5}^{\text{o}}$ .