Third Fourth and Mean Proportions || Lesson 15.5 || Aptitude & Reasoning || Learning Monkey ||

Third Fourth and Mean Proportions
In this class, we discuss Third Fourth and Mean Proportions.
The reader should have prior knowledge of ratios. Click Here.

Proportion: The equality of two ratios is called proportion.
a: b = c: d is written as a: b:: c: d
a, b are extremes.
b, c are mean terms.
Product of means = product of extremes
Example:
10: 1 = 20: 2
Product of extremes = 10 * 2 = 20
Product of means = 1 * 20 = 20
Fourth Proportion:
If a: b = c: d, then d is the fourth proportion to a, b, and c.
Third proportion:
If a: b = b: c, then c is the third proportion to a, b.
Mean proportion or geometric mean:
The mean proportion or geometric mean of a,b is √ab
Assume the mean proportion of a, b is x
x satisfies a/x = x/b
a: x = x: b
product of means = product of extremes.
ab = x^2
x = √ab

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