# Identities of cos

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

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

Daniel.

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...

merci beaucoup