Given: triangle pqr with m∠p=(5x)° , m∠q=(5x)° , and m∠r=(8x)° . prove: x = 10 by the triangle sum theorem, the sum of the angles in a triangle is 180°. therefore, using the given and triangle , . using the substitution property, (5x)°+(5x)°+(8x)°=180° . simplifying the equation gets 18x = 180. finally, using the division property of equality .