tan(π/4) = 1

Lemma: tan π/4 = 1

Proof:

(1) Using definition:

tan(π/4) = sin(π/4)/cos(π/4)

(2) Using the triangle definition of sin and cosine (see here), we know that:

sin (x) = cos (π/2 - x)

(3) This then gives us that:

sin (π/4) = cos(π/2 - π/4) = cos(2π/4 - π/4) = cos(π/4)

(4) So that:

tan(π/4) = cos(π/4)/cos(π/4) = 1

QED