Supplementary Angles
Definition of Supplementary Angles

• Supplementary angles are two angles that add up to give a straight angle, 180°.   

Examples of Supplementary Angles

• In the example shown, 125° and 55° add up to give 180°, so they are called supplementary angles.

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Solved Example on Supplementary Angles

∠A and ∠B are Supplementary Angles and ∠A = ∠B. What are the measures of ∠A and ∠B?
Choices:                    
A. 60°,120°
B. 180°, 180°
C. 45°, 45°
D. 90°, 90°              
Correct Answer: D
Solution:
Step 1: Supplementary Angles are two angles that add up to give a straight angle, 180°.   
Step 2: Given ∠A and ∠B are Supplementary Angles and m∠A = m∠B.
Step 3: Let m∠A = m∠B = x,  then m∠A + m∠B = 180°
Step 4: x + x = 180° [Substitute ∠A = ∠B = x in step 2.]
Step 5: 2x = 180°   
Step 6: alt="" />           [Divide by 2.]   
Step 7: So, m∠A = m∠B = 90°

Related Terms for Supplementary Angles

• Angle
• Straight Angle

Additional Links for Supplementary Angles

• Click here for samples
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