PSAT Math Grid-Ins Question 20: Answer and Explanation
Question: 20
Note: Figure not drawn to scale.
In triangle ABC above, if AB = BC, what is the value of x? (Disregard the degree symbol when gridding your answer.)
Correct Answer: 100
Explanation:
100
The question asks for the value of an angle on a figure. Use the geometry basic approach. Start by labeling the figure with the given information. Because AB = BC, ABC is an isosceles triangle, which has two sides that are equal. In an isosceles triangle, angles that are opposite equal sides must be equal. Therefore, ∠A and ∠C have the same measure, so ∠A is also 40°. ∠A and ∠C have a combined measure of 80°. The sum of the angles inside a triangle must equal 180°. Subtract 80 from 180, and x must measure 100°. The correct answer is 100.
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