Which is correct?(a) cos 3x = 3cosx – 4cos^3x(b) cos 3x = 4cosx – 3cos^3x(c) cos 3x = 3cos^3x – 4cosx(d) cos 3x = 4cos^3x – 3cosxI got this question by my school teacher while I was bunking the class.This is a very interesting question from Trigonometric Functions of Sum and Difference of Two Angles-2 topic in division Trigonometric Functions of Mathematics – Class 11

Which is correct?(a) cos 3x = 3cosx – 4cos^3x(b) cos 3x = 4cosx –

1.	Which is correct?(a) cos 3x = 3cosx – 4cos^3x(b) cos 3x = 4cosx – 3cos^3x(c) cos 3x = 3cos^3x – 4cosx(d) cos 3x = 4cos^3x – 3cosxI got this question by my school teacher while I was bunking the class.This is a very interesting question from Trigonometric Functions of Sum and Difference of Two Angles-2 topic in division Trigonometric Functions of Mathematics – Class 11
Answer» The correct option is (d) cos 3X = 4cos^3x – 3cosx Easy explanation: cos 3x = cos (2x +x) = cos 2x cos x – sin 2x sin x = (2COS^2 x – 1) cos x – 2SIN x cos x sin x = (2cos^2 x – 1) cos x – 2cos x (1 – cos^2 x) = 2cos^3 x – cos x – 2cos x + 2 cos^3 x = 4cos^3 x – 3cos x.

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