Question

How do you use double angle formula to determine cos 120?

Answer 1

Double Angle formula for Cosine (one version):
`color(white)("XXXX")` `cos(2x) = 2cos^2(x)-1`

`cos(120^@)`
`color(white)("XXXX")` `=cos(2*60^@)`

`color(white)("XXXX")` `= 2* cos^2(60^@) -1`

`color(white)("XXXX")` `= 2 * (1/2)^2 -1`

`color(white)("XXXX")` `= -1/2`