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Question
The value of 4 cos3 40° – 3 cos 40° is
Options
`sqrt3/2`
`- 1/2`
`1/2`
`1/sqrt2`
Solution
`- 1/2`
Explanation:
4 cos3 40° – 3 cos 40°
= cos (3 × 40°) [∵ cos 3A = 4 cos3 A – 3 cos A]
= cos 120°
= cos (180° – 60°)
= -cos 60°
= `- 1/2`
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