Question: Error, invalid input

Good Morning.


I have a problem

i have two functions

> xcir(t):=0.3 + 0.15*sin(t):
> ycir(t):= -0.3-0.15*cos(t):
and i suppose that i have a problem in the following equation
> qd2 := ((arctan(-ycir(t)/xcir(t))-arctan(.45*sin(qd3(t))/(.45+.45*cos(qd3(t)))))*180)/(3.1416);
> qd3 := 180*arccos((xcir(t)*xcir(t)+ycir(t)*ycir(t)-.45*(2*.45))/(.45*(2*.45)))/(3.1416);
because the error said "
Error, (in unknown) invalid input: `dsolve/numeric/lexorder` expects its 1st argument, a, to be of type {indexed, name, numeric}, but received arctan((-.3-.15*cos(t))/(.3+.15*sin(t)))"
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