Question

If A, B and C are interior angles of ΔABC, then prove that cos (B+C)/2 = sinA/2

Answer 1

$a + b + c = 180$
b+c=180-a
multiply cos on both sides
then cos b+c =cos (180-a)
then divide both sides by 2
then cos b+c /2=Sina/2

Hence,it is proved