Constant 
Denoted by 
Value 

Euler's number 
$e$ 
~2.71828 
Imaginary unit $i$ 
$i$ 
$\sqrt{1}$ 
Pythagoras' constant 
$\sqrt{2}$ 
~1.41421 
Archimedes' constant 
$π$ 
~3.14159 

$i$ and all the complex numbers can be represented on a two dimensional complex plane in which horizontal axis represents real numbers and
vertical axis represents imaginary axis. 

Area of a circle = $π × radius^2$ 
Perimeter (Circumference) of a rectangle = $2 × π × radius$ 

Area of a circle = $π × radius^2 × C/360$ 
Perimeter of a sector = $(2 × radius) + $ (Length of arc AB) 

Area of a pentagon = $ 5 × (1/2 × height × base)$ 
Perimeter of a pentagon = sum of the lengths of all the five sides 