Highest Common Factors
Highest common factor is the greatest number which will divide exactly into two or more numbers. For example 4 is the highest common factor (HCF) of 20 & 24.
i.e. 20 = 1, 2, (4), 5, 10, 20
24= 1, 2, 3, (4), 6, 8, 12, 24
Example 1:
Find the H.C.F of 24 & 78
Method 1
Express each number as a product of its prime factors
Workings
2 24 2 78
2 12 2 36
2 6 2 18
3 3 3 9
3 3
24=23x3
78=(23 x 3) x 3
The H.C.F. is the product of the common prime factors.
HCF=23x3
=8×3=24
Method II
24=2x2x2x3
78=2x2x2x3x3
Common factor=2x2x2x3
HCF=24
LCM: Lowest Common Multiple
Multiples of 2 are =2,4,6,8,10,12,14,16,18,20,22,24
Multiples of 5 are 5,10,15,20,25,30,35,40
Notice that 10 is the lowest number which is a multiple of 2 & 5.10 is the lowest common multiple of 2& 5
Find the LCM of 20, 32, and 40
Method 1
Express each number as a product of its prime factors
20=22x5
32=25
40=22x2x5
The prime factors of 20, 32 and 40 are 2 & 5 .The highest power of each prime factor must be in the LCM
These are25 and 5
Thus LCM =25 x5
=160
Method II
2 20 32 40
2 10 16 20
2 5 8 10
4 5 4 5
5 5 1 5
1 1 1
LCM =2 x 2 x 2 x 4 x 5 = 160
Class work
Find the HCF of:
(1) 28 and 42
(2) 504 and 588
(3) Find the LCM of 84 & 210
PERFECT SQUARES
A perfect square is a whole number whose square root is also a whole number .It is always possible to express a perfect square in factors with even indices.
9 = 3×3
25= 5×5
225 = 15×15
= 3x5x3x5
= 32 x 52
9216 =96 2
=32 x 32 2
=32 x 42 X 82
=32x24 x26
=32 x2 10
Workings
2 9216
2 4608
2 2304
2 1152
2 576
2 288
2 144
2 72
2 36
2 18
3 9
3 3
9216= 32x210
Example
Find the smallest number by which the following must be multiplied so that their products are perfect square
- 540
- 252
Solution
2 540
2 270
3 135
3 45
3 15 54=22 x 33x 5
5 5
1
The index of 2 even. The index of 3 and 5 are odd .One more 3 and one more 5 will make all the indices even. The product will then be a perfect square .The number required is 3×5 =15
- 2 252
2 126
3 63
3 21
7 7
1
252= 22x32x7
Index of 7 is odd, one more 7 will make it even.
Indices i.e. 22x 32x 72
Therefore 7 is the smallest numbers required
WEEKEND ASSIGNMENT
- The lowest common multiple of 4, 6 and 8 is (a) 24 (b) 48 (c) 12 (d) 40
- Find the smallest number by which 72 must be multiplied so that its products will give a perfect square (a) 3 (b) 2 (c) 1 (d) 5
- The lowest common multiple of 4, 6 and 8 is (a) 24 (b) 48 (c) 12 (d) 40
- The H.C.F. of 8, 24 and 36 is ___ (a) 6 (b) 4 (c) 18 (d) 20
- The L.C.M. of 12, 16 and 24 is ___ (a) 96 (b) 48 (c) 108 (d) 24
THEORY
- Find the smallest number by which 162 must be multiplied so that its product will give a perfect square.
- Find the HCF and L.C.M. of the following figures
30 & 42
64 & 210
See also
WHOLE NUMBERS AND DECIMAL NUMBERS