sin(90-x) = cosx; cos(90-x) = sinx;
tan(90-x) = cotx; cot(90-x) = tanx;
sin(180-x)= sinx; cos(180-x)=-cosx;
tan(180-x)=-tanx; cot(180-x)=-cotx;
sin(180+x)=-sinx; cos(180+x)=-cosx;
tan(180+x)= tanx; cot(180+x)= cotx;
sin(360-x)=-sinx; cos(360-x)= cosx;
tan(360-x)=-tanx; cot(360-x)=-cotx;
sin(-x) =-sinx; cos(-x) = cosx;
tan(-x) =-tanx; cot(-x) =-cotx;
sin(x+y) = sinxcosy + cosxsiny;
sin(x-y) = sinxcosy - cosxsiny;
cos(x+y) = cosxcosy - sinxsiny;
cos(x-y) = cosxcosy + sinxsiny;
tan(x+y) = (tanx + tany)/(1 - tanxtany);
tan(x-y) = (tanx - tany)/(1 + tanxtany);
sin2x = 2sinxcosx = 2tanx/(1 + (tan^2)x);
cos2x = (cos2)x - (sin2)x = 2(cos2)x - 1;
tan2x = 2tanx/(1 - (tan2)x);
2sinxcosx = sin(x+y) + sin(x-y);
2cosxcosy = cos(x+y) + cos(x-y);
2sinxsiny = cos(x-y) - cos(x+y);
sinx + siny = 2sin((x+y)/2)cos((x-y)/2);
sinx - siny = 2cos((x+y)/2)sin((x-y)/2);
cosx + cosy = 2cos((x+y)/2)cos((x-y)/2);
cosx - cosy =-2sin((x+y)/2)sin((x-y)/2);