The correct option (c) [(4c – 3b)/2c]
Explanation:
we know, cosA = [(b2 + c2 – a2)/(2bc)]
∵ a, b, c are in A.P
∴ 2b = a + c
∴ cosA = [{b2 + c2 – (2b – c)2}/(2bc)]
∴ cosA = [{b2 + c2 – (4b2 + c2 – 4bc)}/(2bc)]
= [(– 3b2 + 4bc)/(2bc)]
= [(– 3b + 4c)/2c]
= [(4c – 3b)/2c].