Tutorial 127 |
Multiplying two 2-digit numbers (difference of 4)
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- Choose a 2-digit number.
- Select a number either 3 smaller or 3 larger.
- Add 1 to the smaller number, then square.
- Subtract one from the smaller number.
- Add this to the square.
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Example:
- If the first number is 63, choose 67 as the second number.
- The middle number (the average) is 65.
- Square this middle number: 65 × 65 = 4225. Remember
how to square a 2-digit number ending in 5?
- Subtract 4 from this square: 4225 - 4 = 4221.
- So 63 × 67 = 4221.
See the pattern?
- If the first number is 38, choose 42 as the second number.
- The middle number (the average) is 40.
- Square this middle number: 40 × 40 = 1600.
- Subtract 4 from this square: 1600 - 4 = 1596.
- So 38 × 42 = 1596.
Choose the second number that will give you the easier square, and use your square
shortcuts.
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