Completing the Square
(KS4, Year 10)
If we have the first two terms in a quadratic equation (the x2 and x terms), we can write it as a squared binomial minus a number:
Understanding Completing the Square Using Equations
Let's look at the patterns in the equations:Halve the Number in Front of the x...
The number in the brackets (2) is half the number in front of the x (4)....Then Square and Subtract It
The number being subtracted from the squared brackets (2) is half the number in front of the x (4) squared (22).Understanding Completing the Square Using Geometry
The area of the square plus the area of the rectangle below equals x2 + 4x:Split the rectangle in half. Instead of one rectangle with an area of 4x, there are two rectangles each with an area of 2x:
Place the two rectangles by the side of the square:
We almost have a square. If we place a small square of area 22 in the space... we will complete the square:
Each side of this larger square is x + 2. Its area is (x + 2)2. By adding up the areas we see that:
x2 + 2x + 2x + 22 = (x + 2)2 | 4 small areas = 1 big area |
x2 + 2x + 2x + 22 = (x + 2)2 | Add like terms |
x2 + 4x + 22 = (x + 2)2 |
x2 + 4x + 22 − 22 = (x + 2)2 − 22 | Subtract 22 from both sides |
x2 + 4x = (x + 2)2 − 22 |
Completing the Square in General
In general, we complete the square as:Worksheet
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