Resonance Plot :

Clear["Global`*"]

Plot3D[(1/((x^2)+(y^2))),{x,-1,1},{y,-1,1}]

RLC Circuit :

Clear["Global`*"]

xr=r;
xc=(1/(w c));
xL=w L;
z=(Sqrt[(xr^2)+(((xL-xc))^2)])

wsol= Solve[D[(z^2),w]==0,w] //Last

values={c->10 (10^(-6)),L->100 (10^(-3)),v->100};
current = (v/z)

w0 = w /.wsol  /. values

current /.values /.{r->20, w->w0}

Plot[current /.values /.{r->{20,10,5}}//Evaluate,{w,0,2000},
PlotStyle->{Red, Green,Blue},
PlotRange->All]

Find w at half max

equation = current == (5/2) /.values /.{r->20}

half=NSolve[equation,w]

points ={#,(5/2)}& /@ (w /.half)//Abs

p1=Plot[{(5/2),current} /.values /.{r->{20}} //Evaluate,{w,0,2000},
PlotStyle->{ Green,Blue},
PlotRange->All]

p2=ListPlot[points,PlotStyle->{Red,PointSize[0.05]}]

Show[p1,p2]

?Epilog