Tutorial 130 |
Multiplying two 2-digit numbers (difference of 10)
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- Choose a 2-digit number.
- Select a number either 10 smaller or 10 larger.
- Find the middle number of the two (the average).
- Square this middle number (multiply it by itself).
- Subtract 25 from this square.
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Example:
- If the first number is 36, choose 26 as the second number (10 smaller).
- The middle number (the average) is 31.
- Square this middle number: 31 × 31 = 961. (Remember how
to square a 2-digit number ending in 1?)
- Subtract 25 from this square: 961 - 25 = 936
(subtract mentally in steps: think 961 - 20 - 5 = 941 - 5 = 936).
- So 36 × 26 = 936.
See the pattern?
- If the first number is 78, you might pick 88 as the second number
(10 larger).
- The middle number (the average) is 83.
- Square this middle number: 83 × 83 = 6889. (Remember how
to square a 2-digit number ending in 3?)
- Subtract 25 from this square: 6889 - 25 = 6864
(subtract mentally in steps: think 6889 - 20 - 5 = 6869 - 5 = 6864).
- So 78 × 88 = 6864.
Remember to subtract in easy steps and pick your number to get an easy square.
With practice you will be a whiz at getting these products.
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