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If cos x=1/2, then cos 3x =_______________(a) 0(b) -1(c) 1/\(\sqrt{2}\)(d) 1This question was posed to me in my homework.Origin of the question is Trigonometric Functions of Sum and Difference of Two Angles-2 topic in section Trigonometric Functions of Mathematics – Class 11

Answer»

The correct CHOICE is (B) -1

To explain I would SAY: We KNOW, COS 3x=4cos^3x – 3cos x

= 4(1/2)^3–3(1/2)

= 4/8 – 3/2 = 1/2 – 3/2 = -1.



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