ACT Preparation / Unit 13 ACT 1207 short lesson, about 20 seconds
The Triangle Angle Sum
Captions available
The idea
The three angles of a triangle add to 180°, a consequence of the parallel postulate.
In practice
Two angles of 50° and 70° leave 180 − 120 = 60° for the third. A right triangle's two acute angles therefore add to 90°.
Where people slip
A triangle can have at most one right or obtuse angle, since two would already reach 180°.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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