Trigonometric function problem cosacos2a - 1 = cos3a cos2acos(n-1) - cos(n-2)a = cosna Please tell me what formula to use | Math enthusiast | Mathematical art

Trigonometric function problem cosacos2a - 1 = cos3a cos2acos(n-1) - cos(n-2)a = cosna Please tell me what formula to use

None of them is right
1、cos3a=cos(2a＋a)=cos2acoa－sin2asina；
2. Similar to the above question, the result of this question is also wrong

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