\[ \frac{\pi}{3} = \frac{180^\circ}{3} = 60^\circ \quad \text{and} \quad \frac{5\pi}{6} = \frac{5 \times 180^\circ}{6} = 150^\circ \] ∴ The sum of the three angles is: \[ 60^\circ + 150^\circ + 90^\circ = 300^\circ \] ∴ The measure of the fourth angle is: \[ 360^\circ - 300^\circ = 60^\circ \] Now converting to radians: \[ 60^\circ = 60 \times \frac{\pi}{180} = \frac{\pi}{3} \] ∴ The fourth angle has a sexagesimal measure of \(60^\circ\) and a circular (radian) measure of \(\frac{\pi}{3}\)