Multiplying by eight is done similar to multiplying by nine except we now double the result after the subtraction and subtract 2 from the left hand digit of the multiplicand.
The Rules For Multiplying By Eight
- First Step: subtract from 10 and double.
- Middle Steps: subtract from 9 and double what you get, then add the neighbor.
- Last Step: subtract two from the left-hand figure of the multiplicand.
An Example - Multiplying 1965 by 8
We will look at the example 1965 x 8.
Again I will setup the expression as before when we were multiplying by nine.
For the first step we subtract 5 from 10, giving us 5 and we double it to 10.
We write down the 0 and carry the 1.
Next we subtract 6 from 9, giving us 3, which doubled is 6. We add the neighbor, 5, to give 11 to which we add the carry from the previous step for a total of 12.
We write down the 2 and carry the 1.
Next we subtract 9 from 9 which is zero, doubled is still zero, adding the neighbor we have 6 then we add the carry for a total of 7.
We write down the 7.
Now we subtract 1 from 8 which is 8, doubling that we get 16. Adding the neighbor, 9, we now have 25.
We write down the 5 and carry the 2.
For the last step we subtract 2 from the neighbor, 1, which is minus 1 then we add the carry, which was 2 giving us a total of 1.
We write down the 1 giving us our final answer of 15270.
These rules will work for all numbers, however when multiplying a single digit number by 8 you would simply omit rule 2 and only use rules 1 and 3.
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