Multiplication Utilizing The Square of Binomial

Now let’s take a look at the Square of Binomial, and how we can utilize this formula to ease multiplication of two digit numbers.

Here is the formula again (a ± b)² = a² ± 2ab + b².  

We want to first break the formula down so that it is easier to read.  Rather than the ± sign, we will break it down using the “+” sign once, and utilizing the “-“sign the other time.

(a + b)² = a² + 2ab + b².  This is what the formula looks like with the plus sign—the sum of two numbers raised to the power of 2. We will replace the ‘a’ and ‘b’ with digits and perform the multiplication as prescribed by the formula.

Let (a=30), for instance and we will let (b=1). So (a + b)² will look like this (30 + 1)².

We know that 30 + 1 = 31. So, what we are doing now is finding the square root of the number 31, or in other words 31 × 31.

The formula says that (a + b)² = a² + 2ab + b². or (30 + 1)² = 30² +( 2 ×30×1) + 1².  

This might look as if it makes the multiplication a bit more complicated, but when you get the hang of it, you will be able to do it mentally without using a pen and paper.

We will do it one segment at a time:

1- 30² or 30×30 = 900

2- (2 ×30×1) = 2 ×30 = 60. And 60 × 1 = 60. So the result of the middle part = 60.

3- The last part b², which is 1².  Or 1 × 1 = 1

Now let’s add 1,2 and 3 together as depicted in the formula:

900 + 60 + 1 = 961. This means that 31 × 31 = 961.

Now we will do the square of different between two numbers. (a - b)² = a² - 2ab + b².  

In this example we will be finding the square of the 29.

We will let a=30 and b=1. We want to stick with the number 30 to keep simple and relevant.

Now (30 – 1) is actually 29, which is the number we want to multiply. Let’s apply the formula now. Notice this time that the signs have changed a bit from the previous one. There is a ‘-‘ sign before the middle (2ab).

This is what it should look like with numbers instead of the letters ‘a’ and ‘b’.

(30 - 1)² = 30² -( 2 ×30×1) + 1² . Now let’s look at the sum of each segment.

1- 30² or 30×30 = 900. Same as above

2- (2 ×30×1) = 2 ×30 = 60. And 60 × 1 = 60. So the result of the middle part = 60.

3- The last part b², which is 1².  Or 1 × 1 = 1

The only difference here is, instead of adding the middle part; we will be subtracting it from the first sum, as shown here (a - b)² = a² - 2ab + b².  So here is the breakdown:

900 - 60 + 1 = 841. This means that 29 × 29 = 841.

If you’ve gotten the hang of it, good for you. If not, we will continue breaking it down in the upcoming post. Stay tuned, and feel free to post any questions that you might have.