You may have seen the Fun Fact on squares ending in 5; Here’s a trick that can help you square ANY number quickly. It’s based on the algebra identity for the difference of squares, but with a twist! Can you figure it out? 542 = 50 * 58...

Continue reading...# lightning arithmetic

# Squaring Numbers between 40 and 60

Here’s a quick way to square numbers between 40 and 60. You know the squares of 40, 50, and 60, right? So let’s show what to do for N=51 through 59. Write N as (50+R) for R between 1 and 9. Then N2 will have first...

Continue reading...# Squares Ending in 5

Give me any 2 digit number that ends in 5, and I’ll square it in my head! 452 = 2025 852 = 7225, etc. There’s a quick way to do this: if the first digit is N and the second digit is 5, then the last 2 digits of...

Continue reading...# Difference of Squares

You have all learned that a2 – b2 = (a + b)(a – b) But perhaps you haven’t thought about how to use this to do fast mental calculations! See if you can guess how this trick can help you do the following in your head: 43 x 3778...

Continue reading...# Multiplication by 11

Multiplication by 11 is easy! To multiply by a 2-digit number add the two digits and place the sum in between! 25 x 11 = 275 31 x 11 = 341 57 x 11 = 627 <– you need to carry the 1! What about a 3-digit...

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