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Expand the following expression
cos x cos 2x cos 3x:
Class: Grade 10

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8cos6 x - 10 cos4 x+ 3 cos2x

3 cos6 x - 10 cos4 x + 8 cos2 x

10 cos6 x - 8 cos4 x + 3 cos2 x

8cos6 x + 10 cos4 x - 3 cos2 x

answer is D.

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Detailed Solution

cos x cos2x cos 3x
= cos x (2 cos2 x-1) (4 cos3 x- 3 cos x)
( ∵ cos 2x = 2 cos2 x-1 cos 3x= 4 cos3 x- 3 cos x)
= (2 cos3 x- cosx ) (4 cos3 x - 3 cos x)
= 8 cos6 x - 6 cos4 x - 4 cos4 x + 3 cos2 x
= 8 cos6 x - 10 cos4 x + 3 cos2 x

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