Two angles whose sum is 180° are called supplementary angles. If a straight line is intersected by a line, then there are two angles form on each of the sides of the considered straight line.
What are supplementary angles?
Two angles whose sum is 180° are called supplementary angles. If a straight line is intersected by a line, then there are two angles form on each of the sides of the considered straight line. Those two-two angles are two pairs of supplementary angles. That means, that if supplementary angles are aligned adjacent to each other, their exterior sides will make a straight line.
ABCD is an inscribed polygon ⇒ Given
m∠ABC = 2(m∠D) ⇒ Inscribed Angle Theorem
m∠CDA = 2(m∠B) ⇒ Inscribed Angle Theorem
m∠ABC + m∠CDA = 360° ⇒ The sum of arcs that make a circle is 360°
2(m∠D) + 2(m∠B) = 360° ⇒ Substitution property
m∠D + m∠B = 180° ⇒ Division Property of Equality
Hence, ∠D and ∠B are supplementary angles
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