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If a, b and A are given in a triangle an...

If a, b and A are given in a triangle and `c_(1), c_(2)` are possible values of the third side, then prove that `c_(1)^(2) + c_(2)^(2) - 2c_(1) c_(2) cos 2A = 4a^(2) cos^(2)A`

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The correct Answer is:
`C_(1)^(2)+ C_(2)^(2)-2C_(1), C_(2)cos A = 4a^(2)cos A`
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