Div[{x,x^2 y},{x,y}]

Integrate[1+x^2,{y,-Sqrt[1-x^2],Sqrt[1-x^2]}]

Integrate[2 (Sqrt[1-(x^2)]) (1+(x^2)),{x,-1,1}]

Integrate[1+x^2,{x,-1,1},{y,-Sqrt[1-x^2],Sqrt[1-x^2]}]

Integrate[Cos[t]^2+Cos[t]^2Sin[t]^2,{t,0,2Pi}]