PROPERTIES OF PROPORTION
About "Properties of proportion"
Properties of proportion :
Students who would like to learn proportion must be aware of their properties.
Because, without knowing the properties, always it is difficult to solve problems using proportion.
Let us look at the properties of proportion.
Properties of proportion
1. An equality of two ratios is called a proportion.
Four quantities a, b, c, d are said to be in proportion if a : b = c : d (also written as a : b :: c : d). That is, if a/b = c/d.
2. The quantities a, b, c, d are called terms of the proportion; a, b, c and d are called its first, second, third and fourth terms respectively.
First and fourth terms are called extremes (or extreme terms). Second and third terms are called means (or middle terms).
3. In a proportion,
product of extremes = product of means.
If a : b = c : d are in proportion, ad = bc.
This is called cross product rule.
4. Three quantities a, b, c of the same kind (in same units) are said to be in continuous proportion.
Now, we can write a, b, c in proportion as given below.
a : b = b : c
Using cross product rule, we have b² = ac
5. If a, b, c are in continuous proportion, then the middle term b is called the mean proportional between a and c, a is the first proportional and c is the third proportional.
Thus, if b is mean proportional between a and c,
then b² = ac or b = √ac
6. In a proportion a : b = c : d, all the four quantities need not be of the same type. The first two quantities should be of the same kind and last two quantities should be of the same kind.
7. Invertendo :
If a : b = c : d, then b : a = d : c
8. Alternendo :
If a : b = c : d, then a : c = b : d
9. Componendo :
If a : b = c : d, then (a+b) : b = (c+d) : d
10. Dividendo :
If a : b = c : d, then (a-b) : b = (c-d) : d
11. Componendo and Dividendo :
If a : b = c : d, then (a+b) : (a-b) = (c+d) : (c-d)
12. Addendo :
If a : b = c : d = e : f =.........., then each of these ratios is equal
(a + c + e + ........) : (b + d + f + ........)
13. Subtrahendo :
If a : b = c : d = e : f =.........., then each of these ratios is equal
(a - c - e - ........) : (b - d - f - ........)
Properties of proportion - Practice problems
Problem 1 :
If a : b = c : d = 2.5 : 1.5, what are the values of ad : bc and a + c : b + d ?
Solution :
In the given proportion a : b and c : d, applying cross product rule, we get
ad = bc
Dividing by bc on both sides, we get
ad/bc = 1
ad/bc = 1/1
ad : bc = 1 : 1
Given : a : b = c : d = 2.5 : 1.5 ------ (1)
In the given proportion a : b and c : d, applying the property addendo, we get
a : b = c : d = (a+b) : (c+d) ------ (2)
From (1) and (2), we get
(a+b) : (c+d) = 2.5 : 1.5
(a+b) : (c+d) = (2.5x10) : (1.5x10)
(a+b) : (c+d) = 25 : 15
(a+b) : (c+d) = (25/5) : (15/5)
(a+b) : (c+d) = 5 : 3
Problem 2 :
If a : 3 = b : 4 = c : 7, then, find the value of (a+b+c) : c
Solution :
In the given proportion a : 3 = b : 4 = c : 7, applying the property addendo, we get
a : 3 = b : 4 = c : 7 = (a+b+c) : (3+4+7)
a : 3 = b : 4 = c : 7 = (a+b+c) : 14
Now, all the above four ratios are equal.
Taking the last two ratios, we get
c : 7 = (a+b+c) : 14
c / 7 = (a+b+c) / 14
14 / 7 = (a+b+c) / c
2 = (a+b+c) / c
or
(a+b+c) / c = 2/1
(a+b+c) : c = 2 : 1
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