b[d1_,d2_]:=(-1+d1+d2)/2 +Sqrt[(-1+d1+d2)^2+d2]/2;
d3[d1_1,d2_]:=b[d1,d2]+d1 (1/(d2/b[d1,d2]+1));
d2=2;
abb=Plot[d3[d1,d2],{d1,0.01,3},AxesLabel->{"d1","d3"},
  ImageSize->{400,300}];