The angles of a triangle always adds up to 180 degrees. Hence, if you have the measures of 2 angles of a triangle, you can find the measure of the third angle by deducting the measures of the first 2 angles from 180 degrees. 30+100= 130 The sum of the measures of the first 2 angles of the triangle is 130 degrees. 180-130= 50 Ans: The measure of the third angle is 50 degrees.