Measurements of Common Angles/Acute Angle

Theorem
An acute angle measures $\theta$, where:
 * $0^\circ < \theta < 90^\circ$

or:
 * $0 < \theta < \dfrac \pi 2$

Proof
An acute angle is defined to be an angle whose measure is between that of a zero angle and a right angle.

A zero angle measures $0$ by definition, and a right angle measures $90^\circ$ or $\dfrac \pi 2$.

Hence the result.