syms r c Dbmix yb(r) delta r_sphere yb_bulk yb_surf
D=diff(yb);


dsolve(diff(r^2*2*c*Dbmix/(yb-2)*D)==0,yb(r_sphere)==yb_surf,yb(delta)==yb_bulk)

simp=simplify(ans)

pretty(simp)