Question 552045
<pre>
{{{drawing(400,200,-1.1,1.1,-.1,1, 

triangle(-1,0,1,0,0,.8390996312), locate(-1,0,B), locate(1,0,C), locate(-.02,.94,A), locate(-.9,.1,"40°") )}}} 

Since AB is congruent to AC, we know that &#5123;ABC is
isosceles.  Therefore we know that its base angles are
congruent and thus have the same measure.  Therefore we
will label the measure of the other base angle C, m&#8736;A = 40°
as well:
{{{drawing(400,200,-1.1,1.1,-.1,1, 
locate(-.9,.1,"40°"),
triangle(-1,0,1,0,0,.8390996312), locate(-1,0,B), locate(1,0,C), locate(-.02,.94,A), locate(.75,.1,"40°") )}}} 

Since we know that the sum of the measures of the three angles of any 
triangle must always equal to 180°, we can write

m&#8736;A + m&#8736;B + m&#8736;C = 180°

m&#8736;A + 40° + 40° = 180°

      m&#8736;A + 80° = 180°

            m&#8736;A = 180° - 80°

            m&#8736;A = 100°

{{{drawing(400,200,-1.1,1.1,-.1,1, 
locate(-.9,.1,"40°"),locate(-.1,.75,"100°"),
triangle(-1,0,1,0,0,.8390996312), locate(-1,0,B), locate(1,0,C), locate(-.02,.94,A), locate(.75,.1,"40°") )}}}

Edwin</pre>