Multiplication tricks
Link to previous multiplication post
7. Squaring other 2-digit numbers
Let’s say you want to square58. Square each digit and write a partial answer. 5×5 = 25. 8×8 = 64. Write down 2564 to start. Then, multiply the two digits of the number you’re squaring together, 5×8=40.
Double this product: 40×2=80, then add a 0 to it, getting 800.
Add 800 to 2564 to get 3364.
This is pretty complicated so let’s do more examples.
32×32. The first part of the answer comes from squaring 3 and 2.
3×3=9. 2×2 = 4. Write down 0904. Notice the extra zeros.It’s important that every square in the partial product have two digits.
Multiply the digits, 2 and 3, together and double the whole thing. 2×3x2 = 12.
Add a zero to get 120. Add 120 to the partial product, 0904, and we get 1024.
56×56. The partial product comes from 5×5 and 6×6. Write down 2536.
5×6x2 = 60. Add a zero to get 600.
56×56 = 2536+600 = 3136.
One more example: 67×67. Write down 3649 as the partial product.
6×7x2 = 42×2 = 84. Add a zero to get 840.
67×67=3649+840 = 4489.
8. Multiplying by doubling and halving
There are cases when you’re multiplying two numbers together and one ofthe numbers is even. In this case you can divide that number by two and multiply the other number by 2. You can do this over and over until you get to multiplication this is easy for you to do.
Let’s say you want to multiply14 by 16. You can do this:
14×16 = 28×8 = 56×4 = 112×2 = 224.
Another example: 12×15 = 6×30 = 6×3 with a 0 at the end so it’s 180.
48×17 = 24×34 = 12×68 = 6×136 = 3×272 = 816. (Being able to calculate that 3×27 = 81 in your head is very helpful for this problem.)
9. Multiplying by a power of 2
To multiply a number by 2, 4,8, 16, 32, or some other power of 2 just keep doubling the product as manytimes as necessary. If you want to multiply by 16 then double the number 4 times since 16 = 2×2x2×2.
15×16: 15×2 = 30. 30×2 = 60.60×2 = 120. 120×2 = 240.
23×8: 23×2 = 46. 46×2 = 92. 92×2 = 184.
54×8: 54×2 = 108. 108×2 = 216. 216×2 = 432.
Practice these tricks and you’ll get good at solving many different kinds of arithmetic problems in your head, or at least quickly on paper. Half the fun is identifying which trick to use. Sometimes more than one trick will apply and you’ll get to choose which one is easiest for a particular problem.
Multiplication can be a great sport! Enjoy.
7. Squaring other 2-digit numbers
Let’s say you want to square58. Square each digit and write a partial answer. 5×5 = 25. 8×8 = 64. Write down 2564 to start. Then, multiply the two digits of the number you’re squaring together, 5×8=40.
Double this product: 40×2=80, then add a 0 to it, getting 800.
Add 800 to 2564 to get 3364.
This is pretty complicated so let’s do more examples.
32×32. The first part of the answer comes from squaring 3 and 2.
3×3=9. 2×2 = 4. Write down 0904. Notice the extra zeros.It’s important that every square in the partial product have two digits.
Multiply the digits, 2 and 3, together and double the whole thing. 2×3x2 = 12.
Add a zero to get 120. Add 120 to the partial product, 0904, and we get 1024.
56×56. The partial product comes from 5×5 and 6×6. Write down 2536.
5×6x2 = 60. Add a zero to get 600.
56×56 = 2536+600 = 3136.
One more example: 67×67. Write down 3649 as the partial product.
6×7x2 = 42×2 = 84. Add a zero to get 840.
67×67=3649+840 = 4489.
8. Multiplying by doubling and halving
There are cases when you’re multiplying two numbers together and one ofthe numbers is even. In this case you can divide that number by two and multiply the other number by 2. You can do this over and over until you get to multiplication this is easy for you to do.
Let’s say you want to multiply14 by 16. You can do this:
14×16 = 28×8 = 56×4 = 112×2 = 224.
Another example: 12×15 = 6×30 = 6×3 with a 0 at the end so it’s 180.
48×17 = 24×34 = 12×68 = 6×136 = 3×272 = 816. (Being able to calculate that 3×27 = 81 in your head is very helpful for this problem.)
9. Multiplying by a power of 2
To multiply a number by 2, 4,8, 16, 32, or some other power of 2 just keep doubling the product as manytimes as necessary. If you want to multiply by 16 then double the number 4 times since 16 = 2×2x2×2.
15×16: 15×2 = 30. 30×2 = 60.60×2 = 120. 120×2 = 240.
23×8: 23×2 = 46. 46×2 = 92. 92×2 = 184.
54×8: 54×2 = 108. 108×2 = 216. 216×2 = 432.
Practice these tricks and you’ll get good at solving many different kinds of arithmetic problems in your head, or at least quickly on paper. Half the fun is identifying which trick to use. Sometimes more than one trick will apply and you’ll get to choose which one is easiest for a particular problem.
Multiplication can be a great sport! Enjoy.