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

Multiplication by 9, 99, 999

• For the multiplication by 9

a x 9 = a x 10 - a

ex: 8.9 ; a = 8= (8.10)-8 = 80 – 8 = 72

ex: 22.9 ; a = 22

= (22.10) - 22

= 220 – 22 = 198

ex: 136.9 ; a = 136

= (136.0) – 136

= 1360 – 136 = 1224

ex: 1234.9 ; a = 1234

= (1234.10) - 1234= 12340 – 1234 = 11106

• For the multiplication by 99

100(a - 1) and (99 - (a - 1))

ex: 8.99 ; a = 8= (8-1) and 99- (8-1)

= 7 and 92 = 792

ex: 22.99 ; a = 22

= 10(22-1) and 99- (22-1)

= 21 and 78 = 2178

ex: 136.9 9; a = 136

= (136-2) ;99- [99-(136-2)]

= 134 and 99-35 = 134 and 64 = 13464

OR ax9 = (a.100) - a

12.99 = 1200 – 12 = 1188

• For the multiplication by 999

a x 9 = a x 1000 – a

Ex: 7.999

= 7.1000 – 7

=7000 – 7

= 6993

Ex: 63.999

= 63000 – 63

= 62937

Ex: 582. 999

= 582000 – 582

= 581418

Ex: 7766.999

= 7766000 – 7766

= 7758234

Any Square

Assume you want to find 87².

Find a simple number nearby - a number whose square could befound relatively easy. In the case of 87 we take 90. To obtain 90, we need toadd 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’ssubtract 4 from 66. We are getting 62.

66² = 70×62 + 4² = 4340 + 16 = 4356

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.

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

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.

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

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

Jung HS Daehyun