# T-89 not giving pi back as cos(-1)

1. Mar 11, 2015

### Tyrion101

I have a cosine calculator problem, and it keeps coming up wrong. So I figured to test it would be to do cosine of -1 which should be pi, either symbolically or numerically but it doesn't come out with either. It either produces cos(1) or .540302. It used to give pi as the symbol or as the number. What did I change?

2. Mar 11, 2015

### sk1105

Are you doing inverse cosine?

$arccos(-1) = \pi$

3. Mar 11, 2015

### Staff: Mentor

You should be using cos-1, the inverse cosine function.

4. Mar 11, 2015

### Tyrion101

Thanks! I'd hate to fail the course because of a stupid mistake like this!

5. Mar 11, 2015

### Staff: Mentor

This mistake suggests a fairly serious flaw in your understanding of these trig functions. The sine and cosine functions never get above 1 or below -1, so it's not possible for the cosine of any real number to be $\pi$.

6. Mar 11, 2015

### symbolipoint

Here is how to make sense very quickly:

$$cos(-1)=π$$ -----------------No. Too big for any cosine value.
and: Is that cosine function input in radians or in degrees?