Ratio and Proportion Key Points

Key Points :

Ratio:

The ratio of two quantities a and b in the same units, is the fraction and we write it as a : b.

In the ratio a : b, we call a as the first term or antecedent and b, the second term or consequent.

Eg. The ratio 5 : 9 represents	5	with antecedent = 5, consequent = 9.
	9

Rule: The multiplication or division of each term of a ratio by the same non-zero number does not affect the ratio.

Eg. 4 : 5 = 8 : 10 = 12 : 15. Also, 4 : 6 = 2 : 3.

 Proportion:

The equality of two ratios is called proportion.

If a : b = c : d, we write a : b :: c : d and we say that a, b, c, d are in proportion.

Here a and d are called extremes, while b and c are called mean terms.

Product of means = Product of extremes.

Thus, a : b :: c : d (b x c) = (a x d).

Fourth Proportional:

If a : b = c : d, then d is called the fourth proportional to a, b, c.

Third Proportional:

a : b = c : d, then c is called the third proportion to a and b.

Mean Proportional:

Mean proportional between a and b is ab.

Comparison of Ratios:

We say that (a : b) > (c : d)	a	>	c	.
	b		d

Compounded Ratio:

The compounded ratio of the ratios: (a : b), (c : d), (e : f) is (ace : bdf).

Duplicate Ratios:

Duplicate ratio of (a : b) is (a² : b²).

Sub-duplicate ratio of (a : b) is (a : b).

Triplicate ratio of (a : b) is (a³ : b³).

Sub-triplicate ratio of (a : b) is (a^1/3 : b^1/3).

If	a	=	c	, then	a + b	=	c + d	.     [componendo and dividendo]
	b		d		a - b		c - d

Variations:

We say that x is directly proportional to y, if x = ky for some constant k and we write, x y.

We say that x is inversely proportional to y, if xy = k for some constant k and

we write, x	1	.
	y