With quadratic expression like x
^{2} + bx, you can complete the square by adding
Complete the square. Fill in the number that makes the polynomial a perfectsquare quadratic.
x^{2} + 8x +
Add
to complete the square.
x^{2} + 8x +
= 
x^{2} + 8x + 

Plug in b=8 




= 
x^{2} + 8x + 

Divide 




= 
x^{2} + 8x + 4^{2} 

Square 




= 
x^{2} + 8x + 16 


This quadratic can be written as a square, (x + 4)^{2}, so it is a perfectsquare quadratic.
The number needed to complete the square was 16.