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Use the Sum Identities to Prove the Identity $\cos \theta+\cos 2 \theta+\cos 3 \theta=(2 \cos \theta+1) \cos 2 \theta$

Question 16

Essay

Use the sum identities to prove the identity: $\cos \theta+\cos 2 \theta+\cos 3 \theta=(2 \cos \theta+1) \cos 2 \theta$

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