Question

Without using calculator, find the value of cos (-765°)

Answer 1

• -I've sign indicates clockwise directions from x axis.

=> cos (2 × 360 + 45) = cos(45°)

• After 2 × 360° clockwise rotation it makes 45° angle in 4th quadrant.

• the value of x in 4th quadrant is +I've. (Cosθ = x)

so cos 45° = - 1/√2

note: cosθ = x , cos(k360 + θ) = cosθ

This is how I calculate 3D ☺ fast and accurate