Explain this problem :

Dear student,

I n △ A B C ∠ A + ∠ B + ∠ C = 180 ° C = 180 ° - (A + B) \frac{C}{2} = 90 ° - \frac{(A + B)}{2} W e h a v e \cos A + \cos B = 4 \sin^{2} (\frac{C}{2}) U \sin g \cos C + \cos D = 2 \cos \frac{C - D}{2} \cos \frac{C + D}{2}

2 \sin A \cos B = \sin (A + B) + \sin (A - B) \frac{\sin A + \sin B}{\sin C} = 2 \frac{a + b}{c} = 2 . . . . . . . . (u \sin g \sin e f o r m u l a) a + b = 2 c t h e r e f o r a, c a n d b a r e i n A P R e g a r d s