we need to prove that..
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Here's the solution.
At the second "=" sign (i.e third line), the formulas used are:
cos C + cos D =
2.cos [(C+D)/2].cos[(C-D/2)]
And, sin C + sin D =
2.sin [(C+D)/2].cos[(C-D/2)]
At the second "=" sign (i.e third line), the formulas used are:
cos C + cos D =
2.cos [(C+D)/2].cos[(C-D/2)]
And, sin C + sin D =
2.sin [(C+D)/2].cos[(C-D/2)]
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