**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 352the total of the two figures between the 2 figures.

32X11

322x11

Write extreme right 2 of 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 p 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.

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 p 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.

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

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 = 4pr3

3

=4 x 10.5 x10.5x10.5 x22

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

3

=4 x 10.5 x10.5x10.5 x22

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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