x=1+1.1;
(* ; suppresses output. *)

x
(* print value of x *)

2.1`
x^2

x*(x+1)^2

(x*(x+1))^2

(* almost define a function f: *)
f[x_]:=sin[x];
f[0]

(* a define a function f: *)
g[x_]:=Sin[x] + 0.1*Cos[50x]
g[10]

Sin[10] (* why? *)

Plot[g[x],{x,-10,10},ColorFunction->"BlueGreenYellow"]

h[x_]:=Sin[x]+x
Plot[h[x]+x^3,{x,-1,1},ColorFunction->"BlueGreenYellow"]

Plot[g[x],{x,-10,10},ColorFunction->"BlueGreenYellow",Axes-> False]

FindRoot[h[y],{y,0}]

v={1,2,3}
a={{1,1,1},{2,2,2},{3,3,3}}
(*multiply: *)
a.v
(* another multiply: *)
v.a

Inverse[a]

a={{2,1,1},{2,3,2},{3,3,4}}
Inverse[a]

(* Numerical *)
Inverse[a]//N

a={{x,y},{x+1,y-1}}
Inverse[a]