Section 1

In[]:=
f[x_]:=x
In[]:=
Plot[f[x],{x,0,10},ImageSize->Small]
Out[]=
2
4
6
8
10
0
2
4
6
8
10

Section 2

In[]:=
f[x_]:=
2
x
In[]:=
Plot[f[x],{x,0,10},ImageSize->Small]
Out[]=
2
4
6
8
10
0
20
40
60
80
100