alleryStart learning →A triangle has angles of 31 degrees and 128 degrees at two of its corners. How large is the angle at the third corner?
A31 degrees
B159 degrees
C11 degrees
D21 degrees
Answer & Solution
Correct answer: D. 21 degrees
1. The three angles of any triangle add up to 180 degrees.
2. Add the known angles: 31 plus 128 equals 159.
3. Subtract from 180: 180 minus 159 equals 21 degrees.
4. 159 degrees is the sum of the two known angles, not the missing one, so that option stops a step early.
_Source: OpenStax Elementary Algebra (CC BY 4.0), Ch 3 "Math Models", section 3.4 Solve Geometry Applications: Triangles, Rectangles, and the Pythagorean Theorem_
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