From the picture, we see that there are two congruent sides. Therefore, following the Isosceles Triangle Theorem, if two sides in a triangle are congruent, then the angles opposite those sides are congruent. So, we know that the angles opposite of the congruent sides have an angle measure of x°. We are given the measure of the angle at the top vertex of 100°. Following the Triangle Sum Theorem, the three interior angles in a triangle must sum up to 180°. Therefore, we can create an Algebraic equation to solve for the two base angles that measure x°.
x+100+x=180
x=the base angle measures since they are congruent.
100+x+x=180
Community Property of Addition
100+2x=180.
Added x+x as like terms to = 2x.
100-100+2x=180-100
Subtraction of 100 from both sides to cancel it out and move it.
2x=80
Subtraction operation
2x/2=80/2
Divided by 2 to cancel it out on both sides and move it.
x=40°.
Therefore, the two base angles (angles opposite of the two congruent sides) are 40°.
2(40)+100=80+100=180.
So x=40