COPTR Stage 1 Practice Test

The sum of interior angles in a triangle is:

90 degrees 360 degrees 180 degrees 270 degrees

Correct Answer:

180 degrees

Explanation:

The sum of the interior angles in a triangle is 180 degrees. A clear way to see why is to draw a line through one vertex parallel to the opposite side. The two angles at the other vertices become corresponding angles with angles along that parallel line, so those three interior angles line up to form a straight angle. In other words, angle A plus angle B plus angle C equals 180 degrees. As a quick check, an equilateral triangle has angles 60°, 60°, and 60°, which also adds up to 180°. The other totals—90°, 270°, or 360°—don’t match how angles in a triangle actually add up in flat (Euclidean) geometry.

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