1.
Decimal Fractions:
Fractions in which denominators are powers of 10 are
known as decimal fractions.
Thus, 1/10 = 1 tenth = .1; 1/100 = 1 hundredth = .01;
99/100 = 99
hundredths = .99; 7/1000 =
7 thousandths = .007, etc.;
2.
Conversion of a Decimal into Vulgar Fraction:
Put 1 in the denominator under the decimal point and
annex with it as many zeros as is the number of digits after the decimal point.
Now, remove the decimal point and reduce the fraction to its lowest terms.
Thus, 0.25 = 25/100 = 1/4 ; 2.008 = 2008/1000 = 251/125.
3.
Annexing Zeros and Removing Decimal Signs:
Annexing zeros to the extreme right of a decimal
fraction does not change its value. Thus, 0.8 = 0.80 = 0.800, etc.
If numerator and denominator of a fraction contain
the same number of decimal places, then we remove the decimal sign.
Thus, 1.84/2.99 = 184/299 = 8/13.
4.
Operations on Decimal Fractions:
i.
Addition and Subtraction of Decimal Fractions: The given numbers are so placed under each other
that the decimal points lie in one column. The numbers so arranged can now be
added or subtracted in the usual way.
ii.
Multiplication of a Decimal Fraction By a Power of 10: Shift the decimal point to the right by as many
places as is the power of 10.
Thus, 5.9632 x 100 = 596.32; 0.073 x 10000 = 730.
iii.
Multiplication of Decimal Fractions: Multiply the given numbers considering them without decimal point. Now,
in the product, the decimal point is marked off to obtain as many places of
decimal as is the sum of the number of decimal places in the given numbers.
Suppose we have to find the product (.2 x 0.02 x
.002).
Now, 2 x 2 x 2 = 8. Sum of decimal places = (1 + 2 +
3) = 6.
.2 x .02 x
.002 = .000008
iv.
Dividing a Decimal Fraction By a Counting Number: Divide the given number without considering the
decimal point, by the given counting number. Now, in the quotient, put the
decimal point to give as many places of decimal as there are in the dividend.
Suppose we have to find the quotient (0.0204 Õ 17).
Now, 204 Õ 17 = 12.
Dividend contains 4 places of decimal. So, 0.0204 Õ
17 = 0.0012
v.
Dividing a Decimal Fraction By a Decimal Fraction: Multiply both the dividend and the divisor by a
suitable power of 10 to make divisor a whole number.
Now, proceed as above.
Thus, 0.00066/0.11 = 0.00066
x 100/0.11 x 100 = 0.066/11 =
.006
5.
Comparison of Fractions:
Suppose some fractions are to be arranged in
ascending or descending order of magnitude, then convert each one of the given
fractions in the decimal form, and arrange them accordingly.
Let us to arrange the fractions 3/5, 6/7 and 7/9 in
descending order.
Now, 3/5 = 0.6,
6/7 = 0.857, 7/9 = 0.777...
Since, 0.857 > 0.777... > 0.6. So, 6/7 > 7/9 > 3/5 .
6.
Recurring Decimal:
If in a decimal fraction, a figure or a set of
figures is repeated continuously, then such a number is called a recurring
decimal.
n a recurring decimal, if a single figure is
repeated, then it is expressed by putting a dot on it. If a set of figures is
repeated, it is expressed by putting a bar on the set.
Thus, 1/3 =
0.333... = 0.3; 22/7 = 3.142857142857.... = 3.142857.
Pure
Recurring Decimal: A decimal
fraction, in which all the figures after the decimal point are repeated, is
called a pure recurring decimal.
Converting
a Pure Recurring Decimal into Vulgar Fraction: Write the repeated figures only once in the numerator and take as many
nines in the denominator as is the number of repeating figures.
Thus, 0.5 = 5/9;
0.53 = 53/99; 0.067 = 67/999, etc.
Mixed
Recurring Decimal: A decimal
fraction in which some figures do not repeat and some of them are repeated, is
called a mixed recurring decimal.
Eg. 0.1733333.. = 0.173.
Converting
a Mixed Recurring Decimal Into Vulgar Fraction: In the numerator, take the difference between the
number formed by all the digits after decimal point (taking repeated digits
only once) and that formed by the digits which are not repeated. In the
denominator, take the number formed by as many nines as there are repeating
digits followed by as many zeros as is the number of non-repeating digits.
Thus, 0.16 = (16 – 1)/ 90 = 15/90 = 1/6; 0.2273 = (2273
– 22)/ 9900 = 2251/9900.
7.
Some Basic Formulae:
i. (a
+ b)(a - b) = (a2 - b2)
ii. (a
+ b)2 = (a2 + b2 + 2ab)
iii. (a
- b)2 = (a2 + b2 - 2ab)
iv. (a
+ b + c)2 = a2 + b2 + c2 + 2(ab +
bc + ca)
v. (a3
+ b3) = (a + b)(a2 - ab + b2)
vi. (a3
- b3) = (a - b)(a2 + ab + b2)
vii. (a3
+ b3 + c3 - 3abc) = (a + b + c)(a2 + b2
+ c2 - ab - bc - ac)
viii. When
a + b + c = 0, then a3 + b3 + c3 = 3abc.
