(1) when x = pi/2 or 3pi/2 tan(x) = infinity
(0,pi/2) (pi/2,3pi/2),(3pi/2,2pi) are three intervals,f(x) increases
f(x) is symmetric about npi
so f(2x) is symmetric about npi/2
(2) when A and B are in (pi/2,pi),tanA < 0 and tanB < 0
tan A < 1/tanB
tanA*tanB > 1
tan(A+B) = (tanA+tanB)/(1-tanAtanB) > 0
therefore A+B must be between pi and 3pi/2