Video Transcript
Is ππ΅πΆ an isosceles
triangle?
Letβs begin by recalling that an
isosceles triangle is a triangle that has two congruent sides. Now we might notice in this figure
that there are two congruent sides marked. Line segments π΄πΆ and π΄π΅ are
congruent. But be careful, because this tells
us that triangle π΄π΅πΆ is an isosceles triangle. And we need to determine if
triangle ππ΅πΆ is isosceles. So letβs see if we can work out
either some side lengths or some angles in this smaller triangle. And to help us, we can use the fact
that triangle π΄π΅πΆ is isosceles.
We know that in an isosceles
triangle, the angles opposite the congruent sides are congruent. These are the base angles in the
isosceles triangle. In triangle π΄π΅πΆ, these base
angles will be the angles π΄πΆπ΅ and π΄π΅πΆ. Letβs see if we can find the
measures of these angles. In any triangle, the angle measures
sum to 180 degrees, so we know that the three angle measures in triangle π΄π΅πΆ β
thatβs the measure of angle π΄πΆπ΅, the measure of angle π΄π΅πΆ, and the measure of
angle πΆπ΄π΅ β will all sum to 180 degrees. And we were given that the measure
of angle πΆπ΄π΅ is 44 degrees. So, by subtracting 44 degrees from
both sides, we have that the measure of our two unknown angles is equal to 136
degrees.
And weβve already noted that these
two angles are congruent, that is, that the measure of angle π΄π΅πΆ is equal to the
measure of angle π΄πΆπ΅. So we can write that two times the
measure of angle π΄πΆπ΅ is 136 degrees. By dividing through by two, we have
that the measure of angle π΄πΆπ΅ is 68 degrees. That means that both angles π΄πΆπ΅
and π΄π΅πΆ have a measure of 68 degrees.
Now letβs consider the triangle
ππ΅πΆ and, in particular, the angle ππΆπ΅. Its measure will be equal to 68
degrees minus 37 degrees, which is 31 degrees. Notice that we were given that the
measure of angle ππ΅πΆ is 31 degrees, so triangle ππ΅πΆ has two congruent angles
of 31 degrees each. The converse of the isosceles
triangle theorem states that if two angles in a triangle are congruent, then the
sides opposite those angles are congruent. And by definition, we would have an
isosceles triangle.
Therefore, we can give the answer
to the question as yes, since we have proved that triangle ππ΅πΆ is an isosceles
triangle.