MATHS TIPS
The 11 Rule
You likely all know the 10 rule (to multiply by 10, just add a 0 behind the number) but do you know the 11 rule? It is as easy! You should be able to do this one in you head for any two digit number. Practice it on paper first!
To multiply any two digit number by 11:
- For this example we will use 54.
- Separate the two digits in you mind (5__4).
- Notice the hole between them!
- Add the 5 and the 4 together (5+4=9)
- Put the resulting 9 in the hole 594. That's it! 11 x 54=594
The only thing tricky to remember is that if the result of the addition is greater than 9, you only put the "ones" digit in the hole and carry the "tens" digit from the addition. For example 11 x 57 ... 5__7 ... 5+7=12 ... put the 2 in the hole and add the 1 from the 12 to the 5 in to get 6 for a result of 627 ... 11 x 57 = 627
Practice it on paper first!
Multiply By 5
To quickly multiply by 5, divide the number in two and then multiply it by 10. Often this can be done quickly in your head
Quick Square
If you need to square a 2 digit number ending in 5, you can do so very easily with this trick. Mulitply the first digit by itself + 1, and put 25 on the end. That is all!
252 = (2x(2+1)) & 25
2 x 3 = 6
625
Multiply by 4
To quickly multiply by four, double the number and then double it again.
Often this can be done in your head.
Multiply three digit number with 7*11*13
What's the answer?
Simply write the number twice.
e.g. 346 × 7 × 11 × 13 = 346346
845 × 7 × 11 × 13 = 845845
Squaring numbers having only ones
(11)2
(111)2
(1111)2
It is very simple just count the number of ‘1’ in the question and write the natural numbers up to that number
in ascending order then in descending order up to 1. So the answer for the above questions are
(11)2 =121
(111)2 =12321
(1111)2 =1234321
