LaTeX.org

good morning,
it's the first time that i use LYX. i met some problem when i want to write a numbered formula(Cntrl+Alt+N)
formula:(f(x)=a+b/c+d*f(P1)+a2+b2/c2+d2*f(P2)).an error appeared missing $ inserted.but formula on line it works very well.

could some one tell me how can i do that.

thanks an ...