By first expanding $$\(\cos (2 \theta+\theta)\)$$, show that $$\(\cos 3 \theta \equiv 4 \cos ^{3} \theta-3 \cos \theta\)$$. .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... .................................................................................................................................................... ....................................................................................................................................................
Exam No：9709_w21_qp_23 Year：2021 Question No：7(a)
Answer:
State \(\cos 2 \theta \cos \theta-\sin 2 \theta \sin \theta\)
Attempt correct relevant identities to express in terms of \(\cos \theta\) only
Confirm \(4 \cos ^{3} \theta-3 \cos \theta\) with sufficient detail
Attempt correct relevant identities to express in terms of \(\cos \theta\) only
Confirm \(4 \cos ^{3} \theta-3 \cos \theta\) with sufficient detail
Knowledge points:
2.3.2.2 the expansions of
2.3.2.3 the formulae for sin 2A,cos 2A and tan 2A
Solution:
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