# Identities of cos

1. Nov 25, 2005

### bob4000

i and the rest of my class are finding it impossible to find the answer to this part of the question:

find the exact value of cos A from secA-cosA=1/2

2. Nov 25, 2005

### dextercioby

Make the substitution $\cos A =x$, then the definition of secant and in the end you should get a quadratic equation in "x".

Daniel.

3. Nov 25, 2005

### finchie_88

What you do is that you multiply everything by cosA, so that you get a quadratic, and then solve that in terms of cosA, to get A, simply inverse cos it, eg

(1/cosA) - cosA = 1/2
-cos^2A + 1 = (1/2)cosA
0 = cos^2A + 0.5cosA -1
Then solve with the quadratic formula

edit: dam,beaten to it...

4. Nov 25, 2005

### bob4000

merci beaucoup