This shortcut can be used to obtain the squares of any number which ends in a digit 1. You can also use the shortcut How to obtain the square of any number.

However, this shortcut will be much faster as the number will be closer to multiples of 10. You can use this shortcut to obtain the squares of the number such as 21, 31, 121 etc

Let us understand the algebraic proof behind this shortcut.

a^{2} = (a – 1 )^{2} + 2a – 1

a^{2} = (a +1 )^{2} + (a + a – 1)

Let us understand the shortcut with an example. Let’s say we want to obtain the square of 21.

The entire steps is illustrated in the following figure.

**Step 1: **First step is to write 31 as 31+1. Obtain 30^{2} = 900.

**Step 2: **Add 30 to the given number 31. You will get 61.

**Step 3: **Add 119 to 900 to get the final answer 961

Let us consider another example. Let’s say we want to obtain the 121^{2}.

Very easy right 🙂

On the similar lines, try to calculate the squares of :

- 61
^{2} - 71
^{2} - 81
^{2} - 141
^{2}

There is another shortcut however, I hope you find it useful. Let’s see how does the square of number which ends in 1 look like.

Let us consider a number which ends with 1. The number would look something like *a1*, where *a* can be any number. Let’s see how the square of this number looks like:

Using the modified formula mentioned in Mathematical Shortcut for squaring numbers,

The squares of any 2-digit number ending in 1 would be of the form a^{2}/2a/1.

So, let us consider an example. Can you mentally calculate 121^{2} ?

So, 121^{2} = 14641.

Very easy right :).

On the similar lines, try to calculate the squares of :

21^{2} = 4/4/1 =441

81^{2} = 64/16/1 =7061

51^{2} = 25/10/1 = 2601

**Exercise: **Try to obtain the squares of the following numbers using this technique:

- 61
- 81
- 91
- 141
- 261

If you have any queries regarding this tutorial, please leave a comment. I would be glad to help