Math K-Plus

Definition Of Supplementary Angles

What Are The Characteristics Of An Supplementary Angles?

When two angles add up to 180 degrees they are called Supplementary Angles. For an example, closely look at the diagram below. We have a straight line which is a 180° angle. This 180° angle is cut in two by ray EF. So now the 180° is divided in to two smaller angles that add up to 180°. Now let us express this definition in geometric math terms. Supplementary angles in geometry have the following characteristics.

First let's start with a Straight Angle: ∠DEC. We already know that Straight Angle are 180°.
Now let us draw a Ray EF. The angle at ∠FEC is 33°.
If we look at the diagram below, the two adjacent angles are ∠DEF and d∠FEC. The sum of these two angles are 180° because they form a Straight Angle.
We know 180° minus ∠FEC equals ∠DEF. ∠FEC equals 33°. So 180° minus 33° equals 147°. So the angle at ∠DEF equals 147°.

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