Show[RegionPlot[And[0≤y≤π,y≤x≤π],{x,0,π},{y,0,π},GridLines->Automatic],Graphics[{PointSize[Large],Red,Line[{{1.2,1.2},{π,1.2}}],Point[{{1.2,1.2},{π,1.2}}],Text["x = x1[y] = y",{1,1.3}],Text["x = x2[y] = π",{2.8,1.1}]}]]

Show[RegionPlot[And[0≤x≤π,0≤y≤x],{x,0,π},{y,0,π},GridLines->Automatic],Graphics[{PointSize[Large],Red,Line[{{1.7,0},{1.7,1.7}}],Point[{{1.7,0},{1.7,1.7}}],Text["y = y1[x] = 0",{2.1,0.1}],Text["y = y2[x] = x",{1.4,1.8}]}]]

(∫_0^π)(∫_y^π)Cos[y](E^(Cos[x]))\[DifferentialD]x \[DifferentialD]y

(∫_0^π)(∫_0^x)Cos[y](E^(Cos[x]))\[DifferentialD]y\[DifferentialD]x

TeXForm["(∫_0^π)(∫_y^π)Cos[y](E^(Cos[x]))\[DifferentialD]x \[DifferentialD]y"]

Show[Plot3D[Cos[y](E^(Cos[x])),{x,0,π},{y,0,π},PlotStyle->{{Opacity[0.2],Red},{Opacity[0.2],LightBlue}},BoundaryStyle->{Thick,Blue},PlotRange->All],Plot3D[0,{x,0,π},{y,0,π},Mesh->False,PlotStyle->{{Opacity[0.2],Red},{Opacity[0.2],LightBlue}},BoundaryStyle->{Thick,Blue}]]