Two triangles share a common vertex. In the first, an exterior angle is 100° with interior opposite angles 40° and 60°. A student claims the third angle of this triangle is 80°. Verify: what is the third interior angle of this triangle?

Question

Accepted Answer

80. Two interior angles are 40 and 60, summing to 100. Third = 180 - 100 = 80°. The claim is correct. (Also third = 180 - exterior 100 = 80.)

Two triangles share a common vertex. In the first, an exterior angle is 100° with interior opposite angles 40° and 60°. A student claims the third angle of this triangle is 80°. Verify: what is the third interior angle of this triangle?

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