+2353 cot(900) = cot(5pi) [from degrees to radians]
cot(5pi) = cot(4pi+pi) = cot(pi) = cos(pi)/sin(pi) = -1/0
Therefore cot(900) is undefined
(If you want to keep it in degrees
cot(900) = cot(720+180) = cot(360+360+180) = cot(180) = cos(180)/sin(180) = -1/0)
Reinout 
