Question

Find cos(840°) and sin(840°). Identify the measure of the reference angle.

Answer 1

Reference angle will be
`60^(\circ)`

`cos(840^(\circ))=-0.5`

`sin(840^(\circ))=0.866`

Explanation:

We have given
`cos(840^(\circ))`

Which can be written as
`cos(840^(\circ))=cos(2\pi +120)`

We know that 120° is present in second quadrant

We know that in second quadrant reference angle is given by
`180-angle`

So reference angle =
`180-120=60^(\circ)`

Now
`cos(840)=cos(2\pi +120)=-0.5`

And
`sin(840)=sin(2\pi +120)=0.866`