I am a secondary school maths teacher and I love to use little tips and tricks to help my students. In this article I explain how to find the square of half numbers and how to do the 9 times tables using your fingers. I find that tricks like this can provide students with a much needed alternative to grabbing the calculator to do every little calculation.
The ability to do mental arithmetic is so important for these students, as they can very easily become reliant on the calculator. The ability to do mental arithmetic can speed up their work and boost their confidence; thereby improving their overall performance.
TIP 1: How to square half numbers:
To find the square of a half number (for example: 2.5, 6.5, etc), you simply multiply the whole number part by the next consecutive whole number and then add a quarter.
So for example:
To find (2.5) squared – Multiply 2 (the whole number part) by 3 (the next consecutive whole number), giving 6. Then add a quarter (0.25), so the answer is 6.25!
Lets try it again:
(7.5) ^2 = (7)(8) + 0.25 = 56.25
(3.5)^2 = 12.25
(5.5)^2 = 30.25
(9.5)^2 = 90.25
Easy isn’t it!
TIP 2: Using your fingers to do the nine times tables:
Hold up your 10 fingers. We are going to use these quickly and easily to work out the nine times tables.
Number your fingers (in your head there is no need to write on your fingers!) 1 to 10, from left to right.
So now, let’s say you want to know what 3 times 9 is:
You put down the third finger from your left (the one numbered 3). While you have this finger down, look at your other fingers. To the left you have 2 fingers up and to the right you have 7 fingers up. Putting these together gives 27, so 3 times 9 is 27!
If you want to know what 7 times 9 is: Put down the 7th finger. This leaves 6 fingers raised to the left and 3 fingers raised to the right, so 7 times 9 is 63.
I have found this to be a quick and easy way for students to get to grips with their 9 times tables.
I hope these tips are of some use to you.