Determining Supplementary Angles
If ∡ABC and ∡ABD are supplementary and m∡ABC is five times larger than m∡ABD, determine m∡ABC and m∡ABD (Fig. 8.12).
Let m∡ABD = x, then m∡ABC = 5x. Since these angles are supplementary, we have
m∡ABD + m∡ABC = 180°
x + 5x = 180°
6x = 180°
x = 30°
Thus, m∡ABD = 30° and m∡ABC = 5(30°) = 150°. Note that 30° + 150° = 180°, which is what we expected.