trigonometry question (double angle formula)

tan(a+b) = (tan(a) + tan(b))/(1 - tan(a)tan(b))

tan(a-b) = (tan(a) - tan(b))/(1 + tan(a)tan(b))

 

tan(pi/4) = 1

 

tan(pi/4 + theta)*tan(pi/4 - theta) = (1+tan(theta))/(1 - tan(theta)) * (1 - tan(theta))/(1 + tan(theta)) → 1