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

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

$\Delta RHM$ is acute and equilateral

CORRECT

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

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

$\Delta RHM$ is acute and scalene

$\Delta RHM$ obtuse and scalene

The figure referenced is below.

Rhombus

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

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

A diagonal of a rhombus bisects its angles, so

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

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

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

How to find the length of the diagonal of a rhombus