- Why the identity holds
- Squares and special angles
- Cosine from sine, and back
- Tangent, and why the quadrant
- Secant, cosecant, cotangent
- Quotients and 1 + tan²θ = sec²θ
- Simplify and prove
- Negative angles: odd and even
- Full turns, π − x and π + x
- Unlocks after
- Opens up
- The end of this path.