Definition
Triangle angle sum
In a flat triangle, the three interior angles add to 180°.
What it means
Picture lifting the three corner angles and placing them side by side. They fit into one straight angle. That is why, when two angles are known, you can add them and subtract their total from 180° to find the third.
Remember it
Three inside angles make one straight angle: 180° altogether.
Example
A triangle has angles 47° and 68°. Their total is 115°, so the missing angle is 180° − 115° = 65°.
Watch for
Use the three interior angles—the angles inside the triangle—not an exterior angle beside it.
Common mix-up
The 180° rule adds angle measures, not side lengths.
Exact meaning
The triangle angle sum is the property that the measures of the three interior angles of any non-degenerate Euclidean triangle total 180°.