1. 1. Multiplication by 9, 99, 999
2. Any Square
3. Product of numbers close to
100
4. Multiplication and Division by
5
5. Tips to Improve Maths
Calculation Skill

2. Multiplication by 9, 99, 999
• For the multiplication by 9 • For the multiplication by 99 • For the multiplication
by 999
a x 9 = a x 10 - a 100(a - 1) and (99 - (a - 1))
a x 9 = a x 1000 – a
ex: 8.9 ; a = 8 ex: 8.99 ; a = 8
= (8.10)-8 = 80 – 8 = 72 = (8-1) and 99- (8-1) Ex: 7.999
= 7 and 92 = 792 = 7.1000 – 7
ex: 22.9 ; a = 22 =7000 – 7
= (22.10) - 22 ex: 22.99 ; a = 22 = 6993
= 220 – 22 = 198 = 10(22-1) and 99- (22-1)
= 21 and 78 = 2178 Ex: 63.999
ex: 136.9 ; a = 136 = 63000 – 63
= (136.0) – 136 ex: 136.9 9; a = 136 = 62937
= 1360 – 136 = 1224 = (136-2) ;99- [99-(136-2)]
= 134 and 99-35 Ex: 582. 999
ex: 1234.9 ; a = 1234 = 134 and 64 = 13464 = 582000 – 582
= (1234.10) - 1234 = 581418
= 12340 – 1234 = 11106 OR ax9 = (a.100) - a
12.99 = 1200 – 12 = 1188 Ex: 7766.999
= 7766000 – 7766
= 7758234

3. Any Square
 Assume you want to find 87².
Find a simple number nearby - a number whose square could be
found relatively easy. In the case of 87 we take 90. To obtain 90, we need to
add 3 to 87; so now let's subtract 3 from 87. We are getting 84.
87² = 90×84 + 3² = 7200 + 360 + 9 = 7569
 Assume you want to find 66²
Nearby 66 is 70. To obtain 70 we need to add 4 to 66; so now let’s
subtract 4 from 66. We are getting 62.
66² = 70×62 + 4² = 4340 + 16 = 4356

4. Product of numbers close to 100
Say, you have to multiply 94 and 98. Then 100 - 94 = 6 and 100 - 98 = 2. After that
98 – 6 = 92 and 6x2 = 12. So we can find that 94×98 = 9212.
Another example, 93 and 96. First 100 – 93 = 7 and 100 – 96 = 4. Second 96 – 7 =
89 and 7x4 = 28. Se the result from 93x96 is 8928.
Say, you have to multiply 91 and 92. Then 100 - 91 = 9 and 100 - 92 = 8. After that
92– 9 = 83 and 9x8 = 72. So we can find that 91×92 = 8372.
Another example, 94 and 94. First 100 – 94 = 6. Second 94 – 6 = 88 and 6x6 = 36.
So the result from 94x94 is 8836.

5. Multiplication and Division by 5
Multiplication by 5
It's often more convenient instead of multiplying by 5 to multiply first by 10 and
then divide by 2.
For example,
> 137×5 = 1370/2 = 685
> 123x5 = 1230/2 = 615
> 2468x5 = 24680/2 = 12340
Division by 5
Similarly, it's often more convenient instead to multiply first by 2 and then divide by 10.
For example,
> 1375/5 = 2750/10 = 275
> 1310/5= 2620/10 = 262
> 25060/5 = 50120/10 = 5012

6. Tips to Improve Maths Calculation
Skill
1. Start easy.
2. Learn the multiplication tables and find the patterns in them.
4.Use your fingers.
5.Learn shortcuts.
6. Practise regularly.
7. Don't give up too soon.
8. Challenge yourself.
9. Don't be afraid to reach for a calculator to check your answer if
you're unsure.
10. Perseverance is key. This will take practice, so don't give up too
soon.

7. Assignments
• SOLVE WITH ITS FAST FORMULAS I
1. 79x9 and 296x9 and 7688x9
2. 79x99 and 269x99 and 7688x99
3. 79x999 and 269x999 and 7688x999
• SOLVE WITH ITS FAST FORMULAS II
1. 73² and 127²
2. 63^3 and 120^3
• SOLVE WITH ITS FAST FORMULAS III & IV Jung HS Daehyun
1. 92x99 and 97x95 and 98x97
2. 816x5 and 916x5 and 4168x5
3. 6230/5 and 610/5 and 13480/5
Due: April 22 2013 via PM