Prove that cos 6x = 32 cos6 x – 48cos4 x +18 cos2 x – 1
To prove cos 6x = 32 cos6 x – 48cos4 x + 18 cos2 x – 1
RHS = 32 cos6 x –48cos4 x +18 cos2 x – 1
LHS = cos 6x
= cos 3(2x)
= 4 cos3 2x – 3 cos 2x [cos 3A = 4 cos3 A – 3 cos A]
= 4 [(2 cos2 x – 1)3 – 3 (2 cos2 x – 1) [cos 2x = 2 cos2 x – 1]
= 4 [(2 cos2 x)3 – (1)3 – 3 (2 cos2 x)2 + 3 (2 cos2 x)] – 6cos2 x + 3
= 4 [8cos6 x – 1 – 12 cos4 x + 6 cos2 x] – 6 cos2 x + 3
= 32 cos6 x – 4 – 48 cos4 x + 24 cos2 x – 6 cos2 x + 3
= 32 cos6 x – 48 cos4 x + 18 cos2 x – 1
∴ LHS = RHS
Hence, Proved.