Forward rate equation

I understand the equation for a 1F2 (as an example) (1+S3)^3 = (1+S2)^2 x (1F2) (1+S3)^3 / (1+S2)^2 = 1F2 but what about a 2F1?

Is this from CFAI Test #4? Because I have the same question. For some reason, this stuff just never penetrated into my brain.

has been discussed some days ago… should be self-explanatory…,635491,635524#msg-635524