ClearAll["Global`*"]

stC[x_, k_] := 0.05  x^3 - 0.8  x^2 + (8 - k) x + 2  k^2

stC[x, k]

smC[x_, k_] := Evaluate[D[stC[x, k], x]]

smC[x, k]

\(smCg[k_, tn_] := 
      Plot[smC[x, k], {x, 0, 15}, PlotRange -> {0, 10}, 
        PlotStyle -> {Thickness[tn], RGBColor[1, 0, 0]}, 
        AspectRatio -> 1, 
        PlotLabel -> {"短期限界費用関数 k=", k}, 
        AxesLabel -> {"x (生産量)", "SMC(x,k)"}]\)

smg1 = smCg[0, 0.015]

smg2 = smCg[1.5, 0.01]

smg3 = smCg[3, 0.005]

Show[{smg1, smg2, smg3}, 
      PlotLabel -> "\:77ed\:671f\:9650\:754c\:8cbb\:7528\:95a2\:6570 \
k=0,1.5,3", 
      AxesLabel -> {"x (\:751f\:7523\:91cf)", "SMC(x,k)"}]

saC[x_, k_] := Evaluate[Simplify[stC[x, k]/x]]

saC[x, k]

\(saCg[k_, tn_] := 
      Plot[saC[x, k], {x, 0, 15}, PlotRange -> {0, 10}, 
        PlotStyle -> Thickness[tn], AspectRatio -> 1, 
        PlotLabel -> {"短期平均費用関数 k=", k}, 
        AxesLabel -> {"x (生産量)", "SAC(x,k)"}]\)

sag1 = saCg[0, 0.015]

sag2 = saCg[1.5, 0.01]

sag3 = saCg[3, 0.005]

Show[{sag1, sag2, sag3}, 
      PlotLabel -> "\:77ed\:671f\:5e73\:5747\:8cbb\:7528\:95a2\:6570 \
k=0,1.5,3", 
      AxesLabel -> {"x (\:751f\:7523\:91cf)", "SAC(x,k)"}]

\(Show[{smg1, smg2, smg3, sag1, sag2, sag3}, 
      PlotLabel -> "SMC(x,k),SAC(x,k),k=0,1.5,3", 
      AxesLabel -> {"x (生産量)", "各費用"}]\)

tblsaC1 = Table[saC[x, k], {k, 0, 3, 0.5}]

\(saClisg1 = 
      Plot[Evaluate[tblsaC1], {x, 0, 15}, PlotRange -> {0, 10}, 
        AspectRatio -> 1, 
        PlotLabel -> 
          FontForm["短期平均費用関数群 (0≦k≦3) k を0.5ずつ増加", {"Times-Roman", 9.5}], 
        AxesLabel -> {"x", "SAC(x,k)"}]\)

tblsaC2 = Table[saC[x, k], {k, 0, 3, 0.1}]

\(saClisg2 = 
      Plot[Evaluate[tblsaC2], {x, 0, 15}, PlotRange -> {0, 10}, 
        AspectRatio -> 1, 
        PlotLabel -> 
          FontForm["短期平均費用関数群 (0≦k≦3) k を0.1ずつ増加", {"Times-Roman", 9.5}], 
        AxesLabel -> {"x", "SAC(x,k)"}]\)

sol = Solve[D[stC[x, k], k] == 0, k][[1]]

ltC[x_] := Evaluate[stC[x, k] /. sol]

Expand[ltC[x]]

laC[x_] := Evaluate[ltC[x]/x]

Expand[laC[x]]

laCg = 
      Plot[laC[x], {x, 0, 15}, PlotRange -> {0, 10}, 
        PlotStyle -> {Thickness[0.02], RGBColor[0, 1, 0]}, 
        AspectRatio -> 1, 
        PlotLabel -> "\:9577\:671f\:5e73\:5747\:8cbb\:7528\:95a2\:6570 \
LAC(x)", 
        AxesLabel -> {"x (\:751f\:7523\:91cf)", "LAC(x)"}]

Show[{saClisg1, laCg}, 
      PlotLabel -> "SAC(x,k) \:7fa4\:3068 LAC(x)", 
      AxesLabel -> {"x", "\:5404\:8cbb\:7528"}]

lmC[x_] := Evaluate[D[ltC[x], x]]

lmC[x]

lmCg = 
      Plot[lmC[x], {x, 0, 15}, PlotRange -> {0, 10}, 
        PlotStyle -> {Thickness[0.02], RGBColor[0, 1, 1]}, 
        AspectRatio -> 1, 
        PlotLabel -> "\:9577\:671f\:9650\:754c\:8cbb\:7528\:95a2\:6570 \
LMC(x)", 
        AxesLabel -> {"x (\:751f\:7523\:91cf)", "LMC(x)"}]

Show[{laCg, lmCg}, 
      PlotLabel -> "LAC(x) [\:7dd1]\:3068 LMC(x) [\:6c34\:8272]", 
      AxesLabel -> {"x", "\:5404\:8cbb\:7528"}]

Show[{saClisg1, laCg, lmCg}, 
      PlotLabel -> "SAC(x,k)\:7fa4, LAC(x)[\:7dd1]\:3068LMC(x)[\:6c34\
\:8272]", AxesLabel -> {"x", "\:5404\:8cbb\:7528"}]

tblsmC1 = Table[smC[x, k], {k, 0, 3, 0.5}]

\(smClisg1 = 
      Plot[Evaluate[tblsmC1], {x, 0, 15}, PlotRange -> {0, 10}, 
        PlotStyle -> {RGBColor[1, 0, 0]}, AspectRatio -> 1, 
        PlotLabel -> 
          FontForm["短期限界費用関数群 (0≦k≦3) k を0.5ずつ増加", {"Times-Roman", 9.5}], 
        AxesLabel -> {"x", "SMC(x,k)"}]\)

\( (*\ 短期平均費用関数\ SAC \((x, 
          k)\)\ 群の描画と長期平均費用関数\ LAC \((x)\)\ の描画と短期平均費用関数\ SMC \((x, 
          k)\)\ 群の描画と長期限界費用関数\ LMC \((x)\)\ の描画とを合成\ *) \)

Show[{saClisg1, laCg, smClisg1, lmCg}, 
      PlotLabel -> 
        FontForm["SAC(x,k)\:7fa4,LAC(x)[\:7dd1],SMC(x,k)\:7fa4[\:8d64],LMC(\
x)[\:6c34\:8272]", {"Times-Roman", 9}], 
      AxesLabel -> {"x", "\:5404\:8cbb\:7528"}]

Table[
      Plot[{smC[x, k], lmC[x], saC[x, k], laC[x]}, {x, 0, 15}, 
        PlotRange -> {0, 10}, AspectRatio -> 1, 
        PlotStyle -> {{Thickness[0.01], Hue[0.5]}, {Thickness[0.02], 
              Hue[0.5]}, {Thickness[0.01], Hue[0.3]}, {Thickness[0.02], 
              Hue[0.3]}}, 
        PlotLabel -> 
          FontForm["SAC(x,k)\:7fa4,LAC(x)[\:7dd1], SMC(x,k)\:7fa4,LMC(x)[\
\:6c34\:8272]", {"Times-Roman", 10}], 
        AxesLabel -> {"x", "\:5404\:8cbb\:7528"}], {k, 0, 3, 
        0.5}]