Question

Why radians and staradian are supplimentry units?

Answer 1

Both are derived unit. The classification of supplementary units if not used anymore. They're both used in physics, but measure different thins. Radians measure two dimensional angles. Steradians measure three dimensional projections of angles.  They're not cast-outs , as there are many frequently used named units derived from SI, it's just convention. It's a lost easier to say steradian than to say m^2/m^2 (meter squared over meter squared) .

Answer 2

because these are dimension less units and also used as a base units that's why