in eg 3

how <boc bcums 180-1/2(180-<bac)

 

Using angle sum property of triangle ABC, you can obtain

∠ABC + ∠ACB + ∠BAC = 180°

So, you will obtain

∠BAC = 180° – (∠ABC + ∠ACB)

 

That is why we have substituted the value ∠BAC as 180° – (∠ABC + ∠ACB).

 

Hope! This will help you.

Cheers!!!