Hi Seb
Better call ist substitute terms in an expression.
ST = (c2 o1y - c1 c3 o1y + c2 o1z + c1 c2 o1z)/((c2 - c1 c3) l1);
CT = (2 c2 o1x - 2 c1 c3 o1x + o1z + c1^2 o1z - c2^2 o1z -
c3^2 o1z)/(2 (c2 - c1 c3) l1);
Sin[Theta]^2 + Cos[Theta]^2 - 1 /. {Sin[Theta] -> ST, Cos[Theta] -> CT}
Robert
