# Math Help - cos-1(0) = 90, so cos 90 = 0 but...

1. ## cos-1(0) = 90, so cos 90 = 0 but...

(I am talking in degrees)
cos-1(0) = 90 so cos 90 = 0 but cos (-270) and cos 270 and cos (-90) are all also equal to 0. So then WHY does the calculator only give 90 when I do cos-1(0)?

I know that cos (theta) = cos (-theta) by the way.

2. Originally Posted by s3a
(I am talking in degrees)
cos-1(0) = 90 so cos 90 = 0 but cos (-270) and cos 270 and cos (-90) are all also equal to 0. So then WHY does the calculator only give 90 when I do cos-1(0)?

I know that cos (theta) = cos (-theta) by the way.
Well technically there's an infinite number of solutions, so it just gives the one nearest to 0.

3. Originally Posted by s3a
(I am talking in degrees)
cos-1(0) = 90 so cos 90 = 0 but cos (-270) and cos 270 and cos (-90) are all also equal to 0. So then WHY does the calculator only give 90 when I do cos-1(0)?

I know that cos (theta) = cos (-theta) by the way.
Read this: Trigonometry: Inverse Cosine and Inverse Sine - CliffsNotes