ok this is my other command window were the values of T and RH have been defined, but i still get the same error code 
% the dew point T & RH values 
T = [20 25 30 35];   RH = (30:10:100);
Td = Dew_point(T,RH);
Not enough input arguments.
Error in Dew_point (line 7)
  f = (a.*T)./(b+T) + log(RH/.100);    Td = b.*f ./(a - f);


