A function is defined as
Why is it necessary for to be a dimensionless quantity ?
(i) is a dimensionless physical quantity since it is represented by an angle viz. equal to arc/radius.
(ii) As is dimensionless, all its powers will also be dimensionless. Here the first term is 1 viz. also dimensionless.
Hence, the L.H.S. must be dimensionless so that all the terms in R.H.S. are also dimensionless.