Angles Additive Unknown

Angles Additive Unknown practice with teacher guidance, common slips, and next-step routes.

Core · Angles Additive Unknown · Independent practice · Scalar Entry

The diagram shows 5 equilateral triangles on one straight baseline: 3 congruent large triangles and 2 congruent small triangles. A, B and C are marked. Without measuring, what is the smallest angle of triangle ABC?

Large triangles match each other. Small triangles match each other.

Smallest Angle Degrees

Smallest Angle Degrees

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Print: Ages 13–15 · 5–10 min · Challenge · Angle reasoning · Independent practice · Value entry

Watch: Watch whether the learner enters 60° from one equilateral source triangle or 90° after finding the right angle at C. The decisive evidence is whether the learner compares all three angles of triangle ABC.

Misread: A 60° response treats a source-triangle angle as the target, while 90° finds angle C but misses the word smallest. “Cannot determine” usually means the learner is treating the exact diagram as a picture to measure.

Curriculum: This capstone geometry task uses equilateral-triangle angle structure, congruence, auxiliary lines, and exact angle reasoning to identify a 30° angle without measurement.