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Lesson 2

This document discusses multiplying numbers by 11. It explains that to multiply any number by 11, you put the total of the two rightmost digits between those digits. For example, 32 x 11 is 352 because 3 + 2 is 5, which is placed between the 3 and 2. It then provides examples of multiplying larger numbers by 11 using this method. The document emphasizes that multiplying by 11 in this way is a useful shortcut that can save time, especially when doing calculations in mensuration. It illustrates how to use this method to quickly find the area of a circle and the volume of a sphere.

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104 views2 pages

Lesson 2

This document discusses multiplying numbers by 11. It explains that to multiply any number by 11, you put the total of the two rightmost digits between those digits. For example, 32 x 11 is 352 because 3 + 2 is 5, which is placed between the 3 and 2. It then provides examples of multiplying larger numbers by 11 using this method. The document emphasizes that multiplying by 11 in this way is a useful shortcut that can save time, especially when doing calculations in mensuration. It illustrates how to use this method to quickly find the area of a circle and the volume of a sphere.

Uploaded by

kawsik
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Lesson2
Multiplying a number by 11

To multiply any number by 11 we just put


the total of the two figures between the 2 figures.
32X11
Here first we write 2 of 32 extreme right ,now add up 3+2 =5 ,put 5 before 2 and write 3 extreme left .Required answer is
352
322x11
Write extreme right 2of 322 and add up 2&2 of 322 , 2+2=4 ,write 4 before 2 Now our partial answer is ...........42 .Again
add up 3&2 of 322 .3+2 =5 write before ..42 .Now our partial answer becomes ..542 .Put 3 before 5 . Now our required
answer is 3542

● 77 x 11 = 847

This involves a carry figure because 7 + 7 = 14


we get 77 x 11 = 7147 = 847

Multiplication of 22 ,33 ,44 ,55, 66 , 77 ,88

I think all of you understand this concept . It is a very useful tool ,with the help of this tool
now we learn multiplication of 22 ,33 .44 ,55. 66 . 77 ,88
22x45 =( 2x11)x45 = 11x(2x45)=11x90= 990
33x66 = (3x11)x66 = 11x(3x66)= 11x198= 2178
44x356=(4x11)x356= 11x(4x356)=11x 14 24=1 5664
55x58 =(5x11)x28 = 11x( 5x28) = 11x 140 = 1540
66x23 =(6x11)x23 = 11x(6x23) = 11x138 = 1518
Same way 77 =7x11 and 88=8x11
Multiplication of 11 is great time saver How ???
You can do calculations in Mensuration very fast by this tool
As you know value of π is 22/7 .You can break it as 2x11/7
Ex Find the area of circle of radius of 14 cm

(22/7)x14x14=11x2x2x14=11x56=616square cm
All calculation mentally, no rough work , a great time saver.

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New Page 1

Practice Test 2
Solve mentally these questions
Note down calculation time by your watch
(Suggested time 180 seconds)

11 X 356 11 X 9587 587 X 22


33 X 52 44 X 897 758 X 88
121 X789* 11 X 9874 487 x99
Find the volume of a sphere of radius of 10.5 cm*

*Hints 121 X 789 = 11 X11 X789 = 11 X 8679=95469


volume of sphere = 4πr3
3
= x 10.5 x10.5x10.5 x22
4
3 7
= 4 x0.5 x 10.5x 10.5 x 22
= 2x2 x0.5 x 10.5x10.5 x22
= (2x 10.5)(2x10.5 )(0.5x22)
= 21 x21 x11
= 441 x11
=4851 cube cm
It seems to quite lengthy , actually it takes very little time . It is very tough to calculate 10.5x10.5 x10.5 conventionally.

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