PSAT Math : How to find an angle in a rhombus

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #281 : Plane Geometry

In Rhombus \(\displaystyle RHOM\)\(\displaystyle m \angle H = 45 ^{\circ }\). If \(\displaystyle \overline{RO }\) is constructed, which of the following is true about \(\displaystyle \Delta RHO\)?

Possible Answers:

\(\displaystyle \Delta RHO\) is right and isosceles, but not equilateral

\(\displaystyle \Delta RHO\) is acute and equilateral

\(\displaystyle \Delta RHO\) is right and scalene

\(\displaystyle \Delta RHO\) is acute and scalene

\(\displaystyle \Delta RHO\) is acute and isosceles, but not equilateral

Correct answer:

\(\displaystyle \Delta RHO\) is acute and isosceles, but not equilateral

Explanation:

The figure referenced is below.

Rhombus

The sides of a rhombus are congruent by definition, so \(\displaystyle \overline{RH} \cong \overline{HO}\), making \(\displaystyle \Delta RHO\) isosceles. It is not equilateral, since \(\displaystyle m \angle H = 45 ^{\circ }\), and an equilateral triangle must have three \(\displaystyle 60 ^{\circ}\) angles.

Also, consecutive angles of a rhombus are supplementary - as they are with all parallelograms - so

\(\displaystyle m \angle HRM = 180^{\circ} - m \angle H = 180^{\circ} - 45 ^{\circ} = 135^{\circ}\)

A diagonal of a rhombus bisects its angles, so 

\(\displaystyle m \angle HRO = \frac{1}{2} m \angle HRM = \frac{1}{2} \cdot 135 ^{\circ } = 67\frac{1}{2}^{\circ }\)

Similarly, \(\displaystyle m \angle HOR = 67\frac{1}{2}^{\circ }\)

This makes \(\displaystyle \Delta RHO\) acute.

The correct response is that \(\displaystyle \Delta RHO\) is acute and isosceles, but not equilateral.

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