Consider b‌cos(C+θ)+c‌cos(B−θ) It is simplified as, b‌cos(C+θ)+c‌cos(B−θ)=[b(cos‌C‌cos‌θ−sin‌C‌sin‌θ)+c(cos‌B‌cos‌θ+sin‌B‌sin‌θ)] =[cos‌θ(b‌cos‌C+c‌cos‌B)−sin‌θ(b‌sin‌C−c‌sin‌B)] By projection formula and sine rule,