Math K-Plus

Definition Of Complementary Angles

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What are Complementary Angles?

Complementary Angles in geometry have the following characteristics. When two adjacent angles add up to 90° degrees they are called Complementary Angles. For an example, closely look at the picture below. We have a Right Angles ∠DBC which is a 90° angle. This 90° angle is cut in two by ray BE. So now the 90° is divided in to two smaller angles that add up to 90°. Now let us express this definition in geometric math terms.
  • Another way to say angles are complementary is if two angles next to one another are Right Angles.
  • Complementary angles have length but no width or height.
  • If we look at the diagram below, the two adjacent angles are ∠DBE and ∠EBC. The sum of these two angles are 90° because they form a right angle.
  • Let us assume that angle ∠DBE is 65°. That means that 65° plus ∠EBC equals 90°. Taken to the next step, 90° - 65° = ∠EBC = 25°.
Complementary Angles