Q.In $\triangle ABC$, if $m\angle A = \dfrac{7\pi}{36}^{c}$, $m\angle B = 120°$, find $m\angle C$ in degree and radian.

Step 1: Convert $m\angle A = \frac{7\pi}{36}^{c}$ to degree: $\frac{7\pi}{36}\times\frac{180}{\pi} = 7\times5 = 35°$.

Step 2: Use the angle-sum property of a triangle: $m\angle A + m\angle B + m\angle C = 180°$.

$35° + 120° + m\angle C = 180° \Rightarrow m\angle C = 180° - 155° = 25°$. …

Convert the given radian angle to degree, apply the triangle angle-sum property (180°), …

• Adding angle B (already in degree) to angle A while it is still in radian, without converting first.
• Arithmetic slip in 180° − 155°. …