Question

Question 25 Prove that: cos 6x = 32 cos6 x – 48 cos4 x + 18 cos2 x – 1

Class X1 - Maths -Trigonometric Functions Page 74

Answer 1

LHS = cos6x
= cos3(2x)
use, the formula,
cos3A = 4cos³A - 3cosA

= 4cos³(2x) - 3cos(2x)

use the formula,
cos2A = 2cos²A -1

= 4(2cos²x-1)³ - 3(2cos²x-1)
= 4{8cos^6x -3.(2cos²x)².1 +3.2cos²x.1² - 1 }-6cos²x +3
= 32cos^6x -48cos⁴x + 24cos²x -4 -6cos²x + 3
= 32cos^6x - 48cos⁴x +18cos²x -1 = RHS

Answer 2

Answer:

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