To Prove: 8 cos 3 20° - 6 cos 20° = 1 Taking LHS, = 8 cos 3 20° - 6 cos 20° Taking 2 common, we get = 2(4 cos 3 20° - 3 cos 20°) …(i) We know that, cos 3x = 4cos 3 x – 3 cosx Here, x = 20° So, eq. (i) becomes = 2[cos 3(20°)] = 2[cos 60°] = 1 = RHS ∴ LHS = RHS Hence Proved

Prove that

To Prove: 8 cos³ 20° - 6 cos 20° = 1

Taking LHS,

= 8 cos³ 20° - 6 cos 20°

Taking 2 common, we get

= 2(4 cos³ 20° - 3 cos 20°) …(i)

We know that,

cos 3x = 4cos³x – 3 cosx

Here, x = 20°

So, eq. (i) becomes

= 2[cos 3(20°)]

= 2[cos 60°]

= 1

= RHS

∴ LHS = RHS

Hence Proved