Detailed Answer :
f → force of friction
N →reaction force which will provide centripetal force.
Resolve it into two components – Ncos θ and Nsin θ
Also resolve friction force f into fsin θ and fcos θ.
(centripetal force) mv2/R = fcos θ + Nsin θ (1) (acting towards centre)
Ncos θ = mg + fsin θ or mg = Ncos θ – fsin θ
Dividing eq. (1) by (2)
For maximum speed