HiSET: Math : Angle measure, central angles, and inscribed angles

Study concepts, example questions & explanations for HiSET: Math

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Example Questions

Example Question #71 : Hi Set: High School Equivalency Test: Math

\(\displaystyle \angle 1\) and \(\displaystyle \angle 2\) are a pair of vertical angles.

\(\displaystyle \angle 3\) and \(\displaystyle \angle 4\) are a linear pair.

\(\displaystyle \angle 2\) and \(\displaystyle \angle 3\) are the two acute angles of a right triangle.

Which of the following must be true?

Possible Answers:

\(\displaystyle m \angle 1 = m \angle 4\)

\(\displaystyle m \angle 1 + 90^{\circ }= m \angle 4\)

\(\displaystyle m \angle 1 +m \angle 4 = 90^{\circ }\)

\(\displaystyle m \angle 1 +m \angle 4 = 180^{\circ }\)

\(\displaystyle m \angle 4 + 90^{\circ }= m \angle 1\)

Correct answer:

\(\displaystyle m \angle 1 + 90^{\circ }= m \angle 4\)

Explanation:

\(\displaystyle \angle 1\) and \(\displaystyle \angle 2\) are a pair of vertical angles; it follows that

\(\displaystyle m \angle 1 = m \angle 2\)

\(\displaystyle \angle 3\) and \(\displaystyle \angle 4\) are a linear pair; it follows that they are supplementary - that is,

\(\displaystyle m \angle 3 + m \angle 4 = 180^{\circ }\).

\(\displaystyle \angle 2\) and \(\displaystyle \angle 3\) are the two acute angles of a right triangle; it follows that they are complementary - that is,

\(\displaystyle m \angle 2 + m \angle 3 = 90^{\circ }\).

Therefore, we have the three statements

\(\displaystyle m \angle 1 = m \angle 2\)

\(\displaystyle m \angle 2 + m \angle 3 = 90^{\circ }\)

\(\displaystyle m \angle 3 + m \angle 4 = 180^{\circ }\)

From the second statement, we can subtract \(\displaystyle m \angle 2\) from both sides to get

\(\displaystyle m \angle 2 + m \angle 3 - m \angle 2 = 90^{\circ } - m \angle 2\)

\(\displaystyle m \angle 3 = 90^{\circ } - m \angle 2\)

Substitute this expression for \(\displaystyle m \angle 3\) in the third expression to get

\(\displaystyle 90^{\circ } - m \angle 2 + m \angle 4 = 180^{\circ }\)

Substitute \(\displaystyle m \angle 1\) for \(\displaystyle m \angle 2\):

\(\displaystyle 90^{\circ } - m \angle 1 + m \angle 4 = 180^{\circ }\)

Add \(\displaystyle m \angle 1 - 90^{\circ }\) to both sides:

\(\displaystyle 90^{\circ } - m \angle 1 + m \angle 4 +m \angle 1 - 90^{\circ }= 180^{\circ }+m \angle 1 - 90^{\circ }\)

\(\displaystyle m \angle 4 = 90^{\circ }+m \angle 1\),

or, rearranged,

\(\displaystyle m \angle 1 + 90^{\circ }= m \angle 4\).

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