r1[a_]=0.270834;
r2[a_]=0.0783659;
d3=0.00979574;
g3[a_]:=a (1+d3/a)^2;
d4=0.04365;
g4[a_]:=a (1+d4/a)^2;
abb1=Plot[{r1[a],r2[a],g3[a]},{a,0.000183862,0.521893},
AxesLabel->{"a","r=g(0)"},ImageSize->{400,300}];
abb2=Plot[{g4[a]},{a,0.000183862,0.521893},AxesLabel->{"a","r=g(0)"},
PlotStyle->Dashing[{0.01}],ImageSize->{400,300}];
abb=Show[abb1,abb2,ImageSize->{400,300}];