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Question: In a cyclic quadrilateral ABCD; a, b, c, d denote the length of the sides AB, BC, CD and DA respecti...

In a cyclic quadrilateral ABCD; a, b, c, d denote the length of the sides AB, BC, CD and DA respectively, then cosA is equal to

A
B
C
D
Answer
Explanation

Solution

We have from ∆ ABD

BD2 =

from ∆BDC

BD2 = b2+c2−2bccos⁡Cb ^ { 2 } + c ^ { 2 } - 2 b c \cos C=

Equating the two values we get