1.

The value of 4 cos3 40° – 3 cos 40° is (a) \(\frac{\sqrt{3}}{2}\)(b) \(-\frac{1}{2}\)(c) \(\frac{1}{2}\)(d) \(\frac{1}{\sqrt{2}}\)

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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