L4 · Complementary (90°) and Supplementary (180°) Angles

Learning goal

“I can tell whether two angles are complementary (they compose 90°) or supplementary (they compose 180°), and I can find a missing angle partner.”

Angle — the amount of turn between two sides that meet at a corner, measured in degrees (°).

Complementary angles — two angles that compose 90° - together they make exactly a right angle.

Supplementary angles — two angles that compose 180° - together they make exactly a straight angle (a straight line).

Worked example

Picture the corner of a page - a right angle, 90°. A line drawn from the corner splits it into two angles: 30° and 60°. Step 1: Add the pair: 30 + 60 = 90. Step 2: Two angles that compose 90° are called complementary angles. These two are complementary. Step 3 (finding a missing partner): If one angle is 48°, its complementary partner is 90 - 48 = 42°. Check: 48 + 42 = 90. Say it plainly: complementary is a claim about a SUM. You cannot call a single angle complementary, and you cannot decide without adding.

Check yourself — explain it

Why must you ADD the two angles before calling them complementary?

Paired problem — example still visible

Are 35° and 55° complementary? -> Yes: 35 + 55 = 90

Faded completion — last step is yours

"The complementary partner of 25° is 90 - 25 = ___°" -> 65°

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10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.

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