# Homework Help: Proving Trig Identities

1. Mar 23, 2010

### priscilla98

1. The problem statement, all variables and given/known data
Prove Trig. Identities

1. sec θ (sec θ - cos θ)= tan^2 θ

2. Relevant equations

sec θ = 1/cos θ
tan θ = sin θ/ cos θ
cot θ = cos θ / sin θ

3. The attempt at a solution

1. sec θ * sec θ - sec θ * cos θ

1/ cos θ * 1/ cos θ - 1/ cos θ * cos θ

----> cos θ is crosses out by the right but im confused on 1/cos θ. I know 1/cos θ = sec θ. Wait does this problem refer to the pythagorean identity which is 1 + tan^2 θ = sec^2 θ

2. Mar 23, 2010

### Staff: Mentor

Use = !
Generally you want to start on one side and end up with the expression on the other side.

sec θ * sec θ - sec θ * cos θ = sec2θ - 1 = ?