how can I convert input DC voltage 15+15.sin(w.t) to output DC voltage 15.sine(w.t)?(Exactly Vout=Vin-15)

 

You already have a thread on this.

https://forum.allaboutcircuits.com/threads/circuit.157225/#post-1361444

Stick with that one since this once will only cause side tangents since you can't get "exactly" anything in an analog world.