1. Arithmetic

6 + 3

6.  - 3

1 + 2 3*4 - 6/3

2. Calculator with functions

2^8

N[Pi]
N[Pi, 30]

y1 = Sqrt[x] /. x ->  2
y2 = Sqrt[x] /. x ->  2.0

y1 - y2

20 Log[10, 200.0]

3. Complex numbers

Sqrt[-4]

z = (4 + 3  I)/(2 - I)

Print["real=", Re[z], " imag=", Im[z]]

Abs[z]

Arg[z] // N
%/Degree

Log[z] // N

8 Exp[I 30  °]  // N
% Exp[I 90  °]