Geometry

Forgetting That a Triangle's Angles Sum to 180 Degrees

Students find a missing triangle angle without making all three angles add to 180∘180^\circ.

Quick check

Why students make it

They may focus on the two angles they can see and guess the third from how the triangle looks. Drawings are not always to scale, so visual estimates can be misleading.

The rule done right

The interior angles of every triangle add to 180∘180^\circ. To find a missing angle, subtract the known angles from 180∘180^\circ.

Related resources

trianglesangle-summissing-angles