Complementary and Supplementary Angles

Complementary Angles are angles that, when joined together, form a right angle (90°).

Look at the following example the two adjacent angles below form a right angle when added together. 40° + 50° = 90°.

source: (http://www.mathsisfun.com/geometry/complementary-angles.html)

If only one angle is given then the other angle can be found by subtracting. 90° - the given angle = the other angle. Look at the following example:

source: (Clipart ETC: http://etc.usf.edu/clipart/25700/25799/protractor_25799.htm)

Suppose angle A is 25°. So, to find angle B you would subtract. 90° - 25° = 65°

Supplementary Angles are angles that, when joined together, form a straight line (180°).

Look at the following example the two adjacent angles below form a supplementary angle when added together. 40° + 140° = 180°

source: (http://www.mathsisfun.com/geometry/complementary-angles.html)

Just like with the complementary angles, if only one angle is given then the other angle can be found by subtracting. But using 180°instead of 90°. 180° - the given angle = the other angle. Look at the following example:

source: (Clipart ETC: http://etc.usf.edu/clipart/25700/25799/protractor_25799.htm)

Suppose angle A is 85°. So, to find angle B you would subtract. 180° - 85° = 95°

How can you remember which is which? Easy! Think:

• "C" of Complementary stands for "Corner" (a Right Angle), and
• "S" of Supplementary stands for "Straight" (180 degrees is a straight line)

Now its your turn, try it yourself!

source: (Chichos Caltech: http://www.chicos.caltech.edu/classroom/pointer/diagrams.htm)

If angle ABC = 35

°, how many degrees is angle CBD?

If angle JHK = 87°, how many degrees is JHI?

Source: CSL cartoonstock (http://www.cartoonstock.com/directory/A/Angles.asp)