Answers

2015-09-08T15:09:29+05:30
We know that A+B+C = 180
⇒ A+B = 180-C
⇒(A+B)/2 = (180-C)/2
⇒(A+B)/2 = 90 - C/2

So sin (A+B)/2
= sin (90 - C/2)
= cos (C/2)                   (since sin(90-β) = cosβ)
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