Solving Quadratic Equations: Completing the Square
View the Lesson | MATHguide homepage Updated May 18th, 2017


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    Using the partially completed work below, solve the equation x2 + 10x + 21 = 0 by completing the square.

 x2 + 10x + 21 =   0
Step 1:x2 + 10x            =   
Step 2: x2 + 10x + =   
Step 3: (x + )2 =   
Step 4: x + = + 
Step 5: x = +
Step 6: x1

x2
=

=
- = -7

+ = -3