RE: A trig identity that Mathematica doesn't know?

*To*: mathgroup at smc.vnet.net*Subject*: [mg22242] RE: [mg22172] A trig identity that Mathematica doesn't know?*From*: "Simons, F.H." <F.H.Simons at tue.nl>*Date*: Sat, 19 Feb 2000 01:33:42 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

Gianluca, To show that expr = ArcTan[ y/Sqrt[R^2-y^2] ] - ArcSin[ y/R ] equals 0 for 0<y<R, you might compute the sine of the expression and show that it zero: Simplify[ TrigExpand[ Sin[expr]], R>0] The condition R>0 is necessary! Fred Simons Eindhoven University of Technology