Quote by CompuChip
Did you try the N function?
N[BesselK[1,2]]
BesselK[1,2] // N
In general, Mathematica only "evaluates" such functions for special arguments for which it knows exact values. In all other cases, it leaves the answer exact. You can force it to give a decimal representation using
N[ expr]
or
N[ expr, # of decimals]
Another way that often works is to give the arguments as floating numbers rather than exact values:
BesselK[1., 2]
BesselK[1, 2.]
BesselK[1., 2.]

Thank you CompuChip, but it didn't work, I still get the same expression. Why?
Regards