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The value of 4 cos3 40° – 3 cos 40° is (a) \(\frac{\sqrt{3}}{2}\)(b) \(-\frac{1}{2}\)(c) \(\frac{1}{2}\)(d) \(\frac{1}{\sqrt{2}}\) |
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Answer» (b) \(-\frac{1}{2}\) 4 cos3 40° – 3 cos 40° = cos (3 x 40°) [∵ cos 3A = 4 cos3 A – 3 cos A] = cos 120° = cos (180° – 60°) = -cos 60° = - \(\frac{1}{2}\) |
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