In details

Proportion Properties


1st property

In a proportion, the sum of the first two terms is for the 2nd (or 1st) term, just as the sum of the last two terms is for the 4th (or 3rd).

Demonstration:Consider the proportions:

and

Adding 1 to each member of the first proportion, we get:

Doing the same in the second proportion, we have:

Example:

  • Determine x and y in proportion , knowing that x + y = 84.
    Solution:

Like this:

x + y = 84 => x = 84-y => x = 84-48 => x = 36.

Soon, x = 36 and y = 48.

2nd property

In a proportion, the difference of the first two terms is for the 2nd (or 1st) term, just as the difference of the last two terms is for the 4th (or 3rd).

Demonstration:Consider the proportions:

and

By subtracting 1 from each member of the first proportion, we get:

Doing the same in the second proportion, we have

(Multi 2 members by -1)

Example:

  • Knowing that x-y = 18, determine x and y in proportion .
    Solution:

For the 2nd property, we have to:

x-y = 18 => x = 18 + y => x = 18 + 12 => x = 30.Thus x = 30 and y = 12.

3rd property:

In a proportion, the sum of the antecedents is for the sum of the consequents, just as each antecedent is for its consequent.

Demonstration:Consider the ratio:

By exchanging the means we have:

Applying the 1st property, we get:

By exchanging the means, we finally get:

4th property:

In a proportion, the difference of the antecedents is for the difference of the consequents, just as each antecedent is for its consequent.

Demonstration:Consider the ratio:

By exchanging the means we have:

Applying the 2nd property, we get:

By exchanging the means, we finally get:

Example:

  • Knowing that a-b = -24, determine The and B in proportion .
    Solution:
    For the 4th property, we have to:

5th property:

In a proportion, the product of the antecedent is to the product of the consequent, just as the square of each antecedent is to the square of its consequent.

Demonstration:Consider the ratio:

Multiplying the two members by we have:

Like this:

Note: The 5th property may be extended for any number of reasons. Example:

Next: Multiple Ratio