Question

If `Cos(theta)= 2/3` then what is `cos(theta + pi)`?

Answer 1

`cos(theta+pi)=-2/3`

Explanation:

Use the formula cos(A+B)=cosAcosB-sinAsinB.So
`cos(theta+pi)=cos theta cos pi -sin theta sin pi`

`=2/3 (-1)-sin theta (0)`

Note that we can find the sin of theta from the given ratio but since sine of pi is 0 we don't have to bother finding it.

`=-2/3 -0`

`=-2/3`