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
322
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3542
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.
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 = 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.
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