verifying identities

[on3winyoureyes]

verify it's an identity:
cos(90°+x) = -sinx


My work
cos(90°+x) = -sinx
=cos90°+cosx
=cos90° is 0 so 0+cosx
(cos90)(cosx)-(sin90)(sinx)
=0-sinx
-sinx=-sinx

 

[Deleted member 4993]

verify it's an identity:
cos(90°+x) = -sinx


My work
cos(90°+x) = -sinx
=cos90°+cosx ...................................Incorrect
=cos90° is 0 so 0+cosx
(cos90)(cosx)-(sin90)(sinx)
=0-sinx
-sinx=-sinx

Use:

Cos(A+B) = Cos(A) * Cos(B) - Sin(A) * Sin(B)

Now expand Cos(90° + x) using the formula above.