PSAT Math : How to find the length of the diagonal of a rhombus

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #601 : Geometry

In Rhombus \displaystyle RHOM\displaystyle m \angle R = 60^{\circ }. If \displaystyle \overline{HM} is constructed, which of the following is true about \displaystyle \Delta RHM?

Possible Answers:

\displaystyle \Delta RHM is obtuse and isosceles, but not equilateral

\displaystyle \Delta RHM obtuse and scalene

\displaystyle \Delta RHM is acute and isosceles, but not equilateral

\displaystyle \Delta RHM is acute and scalene

\displaystyle \Delta RHM is acute and equilateral

Correct answer:

\displaystyle \Delta RHM is acute and equilateral

Explanation:

The figure referenced is below.

Rhombus

Consecutive angles of a rhombus are supplementary - as they are with all parallelograms - so

\displaystyle m \angle RHO= 180^{\circ} - m \angle R = 180^{\circ} - 60^{\circ} = 120^{\circ}

A diagonal of a rhombus bisects its angles, so 

\displaystyle m \angle RHM = \frac{1}{2} m \angle RHO= \frac{1}{2} \cdot 120^{\circ} = 60^{\circ}

A similar argument proves that \displaystyle m \angle RMH = 60^{\circ}.

Since all three angles of \displaystyle \Delta RHM measure \displaystyle 60^{\circ}, the triangle is acute. It is also equiangular, and, subsequently, equilateral.

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