\[ \frac{\pi}{3} = \frac{180^\circ}{3} = 60^\circ \quad \text{and} \quad \frac{5\pi}{6} = \frac{5 \times 180^\circ}{6} = 150^\circ \] \[ \therefore \text{Sum of the three angles} = 60^\circ + 150^\circ + 90^\circ = 300^\circ \Rightarrow \text{Fourth angle} = 360^\circ - 300^\circ = 60^\circ \] \[ 60^\circ = 60 \times \frac{\pi}{180} = \frac{\pi}{3} \text{ radians} \] Therefore, the measure of the fourth angle is 60° in sexagesimal (degree) form and \(\frac{\pi}{3}\) radians in circular form.