Question

Question #26248

Answer 1

`-0.9`

Explanation:

Rotating an angle by `pi` flips it 180 degrees around the circle.

This means that both its `x` and `y` coordinates are flipped (made negative).

Since `cos` is just the `x` coordinate of an angle on the unit circle, and rotating that angle by `pi` will flip its `x` coordinate (and its `y` too, but that's not important right now) we can say that:

`cos(theta+pi) = -costheta`

Therefore:

`if costheta = 0.9`

`"then" cos(theta+pi) = -0.9`

Final Answer