ContourPlot[
If[(x>0 && x< (π/2))&&(y>0 && y< (π/2)),
 Sin[x+y],
0]
,{x,-2,3},{y,-2,3},AxesLabel->{x,y}]

NIntegrate[Sin[x+y],{x,0,(π/2)},{y,0,(π/2)}]

Integrate[Sin[x+y],{x,0,(π/2)},{y,0,(π/2)}]

xyRnorm:={Random[NormalDistribution[0,1]],Random[NormalDistribution[0,1]]}

ListPlot[Table[xyRnorm,{1000}]]

rad[pnt_]:=(Sqrt[((pnt[[1]])^2)+((pnt[[2]])^2)])

rad[xyRnorm]

rad /@ Table[xyRnorm,{10000}] //Histogram

ListPlot[Table[xyRnorm,{1000}]]

Histogram3D[Table[xyRnorm,{1000}]]

Table[Random[ExponentialDistribution[1]],{10000}]//Histogram