Question

triangle abc, shown in the diagram below, is an isosceles triangle where $overline{ab}congoverline{ac}$. what is the measure, in degrees, of $angle abc$?
$43^{circ}$
$123^{circ}$
$84^{circ}$
$57^{circ}$

Explanation:

Step1: Find $\angle ACB$

$\angle ACB = 180^{\circ}- 123^{\circ}=57^{\circ}$

Step2: Use isosceles - triangle property

Since $\overline{AB}=\overline{AC}$, $\angle ABC=\angle ACB$. So $\angle ABC = 57^{\circ}$

Answer:

E. $57^{\circ}$