# Basic Proportionality Theorem

How to use the basic proportionality theorem to find the segments divided proportionally by parallel lines: 3 formulas, 3 examples, and their solutions.

## FormulaSegements of a Triangle

If a line parallel to the side of a triangle

divides the other two sides

into four segments,

then the divided segments are proportional:

a/b = a'/b'.

This is the basic proportional theorem.

(= Intercept theorem, Thales' theorem)

## Example

By the middle segment

parallel to the bottom side,

two sides of a triangle

are divided into four segments.

Then these divided segments are proportional.

The left segments are

(top) = 5 and (bottom) = 3.

The right segments are

(top) = 6 and (bottom) = x.

So 5/3 = 6/x.

Solve the proportion.

Then 5x = 6⋅3.

6⋅3 = 18

Divide both sides by 5.

Then x = 18/5.

So x = 18/5.

## FormulaSegments of Two Lines (1)

If three parallel lines divide two lines,

then the divided segments are also proportional:

a/b = a'/b'.

If two lines are not parallel,

two lines will meet at a point.

(dashed line)

Then you can see that

this formula is the extended version

of the basic proportionality theorem.

(If two lines are parallel,

then a = a' and b = b'.)

## Example

By three parallel lines,

two lines are divided into four segments.

Then these divided segments are proportional.

The left segments are

(top) = 8 and (bottom) = 16.

The right segments are

(top) = x and (bottom) = 18.

So 8/16 = x/18.

8/16 = 1/2

1/2 = x/18

Solve the proportion.

Then 2x = 18.

Divide both sides by 2.

Then x = 9.

So x = 9.

## FormulaSegments of Two Lines (2)

If the whole segment is given,

use this extended formula:

(a + b)/b = (a' + b')/b'.

## Example

Three vertical lines

are all perpendicular to the bottom line.

Then the vertical lines are all parallel.

(The right angles are

corresponding angles in parallel lines.)

The whole length of the top segment is 11.

So write the whole length of the bottom segment:

6 + 4 = 10.

By three parallel lines,

two lines are divided into four segments.

The top segments are

(whole) = 11 and (right) = x.

The bottom segments are

(whole) = 10 and (right) = 6.

So 11/x = 10/6.

10/4 = 5/2

11/x = 5/2

Solve the proportion.

Then 5x = 11⋅2.

11⋅2 = 22

5x = 22

Divide both sides by 5.

Then x = 22/5.

So x = 22/5.