Question

The value of 4 cos³ 40° – 3 cos 40° is

Options

• `sqrt3/2`

• `- 1/2`

• `1/2`

• `1/sqrt2`

Answer 1

`- 1/2`

Explanation:

4 cos³ 40° – 3 cos 40°

= cos (3 × 40°) [∵ cos 3A = 4 cos³ A – 3 cos A]

= cos 120°

= cos (180° – 60°)

= -cos 60°

= `- 1/2`