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2cos3xcosx = cos2C + cosC
By Rania Kashi (P3223) on Saturday, February 24, 2001 - 02:57 pm:

This question has stumped me and its only worth 2 marks!! What am I missing?? I don't know what to do for it, please help!

2cos3xcosx = cos2C + cosC
Express C in terms of x.

By Kerwin Hui (Kwkh2) on Saturday, February 24, 2001 - 04:35 pm:

We have

2 cos A cos B º cos (A+B) + cos(A-B)

so, substituting A=3x, B=x gives

2 cos 3x cos x º cos 4x + cos 2x

so, since the displayed equation holds for all x, we have C = 2np ± 2x

Kerwin