# Ratio and proportion. Proportionality

*Ratio. Proportion ( border and middle terms ).*

The main property of a proportion.

Proportional values. Factor of a proportionality.

The main property of a proportion.

Proportional values. Factor of a proportionality.

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Ratio
is a quotient of dividing one number by another.
Proportion
– an equality of two ratios.
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For instance:

12 : 20 = 3 : 5;
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a : b = c : d .
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Border terms of the proportion:
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12 and 5 in the first proportion;

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a
*
and
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d
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in the second proportion.

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Middle terms of the proportion:
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20 and 3 in the first proportion;

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b
*
and
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c
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in the second proportion.

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The main property of a proportion:
A product of border terms of a proportion is equal to a product of its middle terms.
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Two mutually dependent values are called
proportional
ones, if a ratio of their values is saved as invariable.
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This invariable ratio of proportional
values is called a

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**factor of a proportionality**

E x a m p l e . |
A mass of any substance is proportional to its volume. For instance,
2 liters of mercury weigh 27.2 kg, 5 liters weigh 68 kg, 7 liters weigh 95.2 kg.
A ratio of mercury mass to its volume ( factor of a proportionality ) will be
equal to:
Thus, a factor of a proportionality in this example is density. |