Most of us have learned by heart the multiplication table up to 10 x 10. A simple technique will enable us to extend our multiplication power up to 20 x 20.
Example 17.1: Compute 14 x 12
- Cover one of the ten’s digit and add what remains to the other number: 14 + 2 or 4 + 12 will both give 16. This will be the first or left hand part of the product.
- Cover both ten’s digit and multiply what remains: 2 x 4 = 8. This will be the second or right hand part of the answer.
- Thus 14 x 12 = 16 | 8 or 168
What we actually did in the first step is to add the excess over the base (10) of a number to the other.In the second step we multiplied the excesses. The algebraic proof of this method, known as base multiplication, is shown below: (x + a) * (x + b) = x2 + (a + b) * x + a * b = (x + a + b) * x + a * b = [ (x + a) + b] * x + a * b |
In our example above, x = 10, a = 4 and b = 2. So we have
14 x 12
= [ (10 + 4) + 2 ] * 10 + 4 * 2
= 160 + 8
= 168
In this case both multiplicands are above the base.
Example 17.2: Compute 16 x 13
(10 + 6) x (10 + 3)
- Add 3 (excess of 13 over 10) to 16 to get 19
= ( 10 + 6) + 3 |
= 19 |
- Multiply the excesses 6 x 3 = 18.
= 19 | 6 x 3
= 19 | 18 - Since there is only one zero in the base, only one place is allotted for the right hand part. Therefore the 1 of 18 must be carried or added to 19 to get a final answer of 208
= 20 | 8 = 208
Example 17.3: Find 107 x 104
- The left part is 107 + 4 = 111, and the right part is 7 * 4 = 28
(100 + 7) * (100 + 4)
= 107 + 4 | 7 x 4
= 111 | 28
= 11,128Note that in this example we did not write the two zeroes after 111 but two places are reserved for the 28.
Example 17.4: Find 1,025 x 1,012
- The left part is (1025 + 12 ) = 1037 and the right part is 25 * 12 = 300
1,025 x 1,012
= (1000 + 25) * (1000 + 12)
= 1025 + 12 | 25 * 12
= 1037 | 300
= 1,037,300
Example 17.5: Find 115 x 111
- The left part is (115 + 11) and the right part 165
115 x 111
= (100 + 15) (100 + 11)
= 115 + 11 | 15 * 11
= 126 | 165 - Since we have only two zeroes in our base, we can allot only two spaces for the right hand part of the answer, so we must “carry” the 1 of 165 into the left side.
= 12,765
Example 17.6: Find 102 x 104
- This is pretty straightforward with the left 102 + 4 = 106 and the right is 2 * 4 = 8
102 * 104
= (100 + 2) (100 + 4)
= 102 + 4 | 2(4)
= 106 | 08 - In this case, the product of the excesses is 8 but since the base is 100, two spaces are allotted to it. We write it as 08.
= 10,608
Example 17.7: 103 x 119
- The left side is 103 + 19 = 3 + 119 = 122 and the right side is 3 * 19 = 57
(100 + 3) * (100 + 19)
= 119 + 3 | 3 * 19
= 12,257 - Note that in this example we chose to add the smaller excess to the other number. This always leads to a simpler calculation.
Exercise 17: Find the following products using base multiplication
- 12 x 13 =
- 14 x 17 =
- 15 x 18 =
- 108 x 101=
- 116 x 102 =
- 108 x 112 =
- 112 x 114=
- 123 x 106 =
- 1021 x 1006 =
- 1432 x 1002 =
Answers to all exercises are found in the answer key.
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