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Home » Express each of the following as an algebraic sum of sines or cosines : (iii) 2cos 7x cos 3x (iv) 2sin 8x sin 2x
Express each of the following as an algebraic sum of sines or cosines : (iii) 2cos 7x cos 3x (iv) 2sin 8x sin 2x
CBSE | Class 11 | Excercise 15C | Maths | RS Aggarwal | Trigonometric, or Circular, Functions
Express each of the following as an algebraic sum of sines or cosines : (iii) 2cos 7x cos 3x (iv) 2sin 8x sin 2x

Answer:

iii) 2cos7xcos3x = cos (7x+3x) + cos (7x – 3x)
=cos10x + cos 4x
Using,
2cosAcosB = cos (A+ B) + cos (A – B)

iv)2sin8xsin2x = cos (8x – 2x) – cos (8x + 2x)
= cos6x – cos10x
Using,
2sinAsinB = cos (A – B) – cos (A+ B)

i

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