Problem:
If ∠CBD is a right angle, then this protractor indicates that the measure of ∠ABC is approximately
Answer Choices:
A. 20∘
B. 40∘
C. 50∘
D. 70∘
E. 120∘
Solution:
Since ∠CBD is a right angle, side BC must cross the protractor at 70∘. Thus the measure of ∠ABC is 70∘−20∘=50∘.
OR
∠ABC =∠ABD−∠CBD = (∠OBD−∠OBA)−∠CBD=(160∘−20∘)−90∘ = 50∘.
Answer: C​.
The problems on this page are the property of the MAA's American Mathematics Competitions
