Se tine cont ca sin(-a) = -sin(a) si cos(-a)=cos(a)
si sin(2kpi + a) = sina si cos(2kpi + a) = cos a, cu a in primul cadran (0, pi/2)
Reducem unghiurile la primul cadran :
sin(-28pi/3) = -sin(7pi + pi/3) = -sin60 = -(rad3)/2
cos(-28pi/3) = cos(7pi + pi/3) = cos60 = 1/2
tg(-28pi/3) = (-sin60)/cos60 = -((rad3)/2)/(1/2) = -rad3
ctg(-28pi/3) = 1/tg(-28pi/3) = 1/(-rad3) = -rad3/3
