দেওয়া আছে, $\cos A + \cos B = \sin C$। $$ \begin{aligned} 2 \cos \frac{A+B}{2} \cos \frac{A-B}{2} &= 2 \sin \frac{C}{2} \cos \frac{C}{2} \\ A+B+C = \pi, \frac{A+B}{2} = \frac{\pi}{2} - \frac{C}{2} \\ \Rightarrow 2 \sin \frac{C}{2} \cos \frac{A-B}{2} &= 2 \sin \frac{C}{2} \cos \frac{C}{2} \\ \Rightarrow \cos \frac{A-B}{2} &= \cos \frac{C}{2} \\ \Rightarrow \frac{A-B}{2} &= \pm \frac{C}{2} \\ \Rightarrow A-B &= C \quad (\lor A-B = -C) \\ \Rightarrow B &= A-C \quad (\lor B = A+C) \end{aligned} $$ $A+B+C = \pi$ সমীকরণে $B = A+C$ বসালে, $2B = \pi \Rightarrow B = \frac{\pi}{2}$।