x^2/. x->3

Manipulate[Plot3D[Sin[x] Sin[y], {x,0, xx}, {y, 0, 7}],{xx,1,20}]

Manipulate[Integrate[x^n,x],{n,0,10}]

f[x_] := Exp[-x^2/2]/Sqrt[2 Pi]

f[0]

N[(1/(Sqrt[2 π]))]

N[(1/(2 π))]

Integrate[f[x], {x,0,Infinity}]

N[(1/(2 (Sqrt[2 π])))]

N[((Erfc[(1/(Sqrt[2]))])/(2 (Sqrt[2 π])))]

Integrate[f[x],{x,1,Infinity}]

N[(1/2) Erfc[(1/(Sqrt[2]))]]

Plot[f[x],{x,-3,3}]

Plot[N[Integrate[f[x],{x,xx,Infinity}]],{xx,-5,5}]