# Straight Lines

Sub Topics

Straight line can be defined as a simple geometrical figure. even though it is simple it is one of the important concept in geometryconcepts . We come across the concept of straight lines in our day to day experience. Sides of all polygons are straight lines. The path of the shortest distance between two points is a straight line.

## Graph of a Straight Line

The graph of a linear function is a straight line. In fact the name ‘linear’ is used in the name because of this fact. The function of a straight line is expressed in several forms depending upon the context. The orientation of a straight line in a grid depicts the nature of a linear function.

A horizontal line represents a function but not a vertical line.

## Ratio in a Straight Line Segment

As a special case of ratio, the coordinates of the mid point of a line segment with end points as (x1, y1) and (x2, y2) are given by,

x = ½( x1 + x2) and y = ½( y1 + y2)

## Area of a Triangle Formed by three Segments of Straight Lines

The area of the triangle ABC

A = Area of trapezoid DABE + Area of trapezoid EBCF - Area of trapezoid DACF

= ½ (y2 + y1)(x2 – x1) + ½ (y3 + y2)(x3 – x2) - ½ (y3+ y1)(x3 – x1)

On simplification, A = ½ [ x1 (y3 – y2) + x2 (y1 – y3) + x3(y2 – y1)]

What happens when the points A, B and C are in the same line (collinear) ?

In such a case A = 0 or, x1 (y3 – y2) + x2 (y1 – y3) + x3(y2 – y1) = 0

This brings an important concept that three points with coordinates (x1, y1), (x2, y2) and (x3, y3) will be collinear if,

x1 (y3 – y2) + x2 (y1 – y3) + x3(y2 – y1) = 0

## Example Problems on Straight Lines

Example 1

The coordinates of points A and B are (-1, -3) and (2, 1) respectively. What is the distance between A and B?

Solution

As per the distance formula,

AB2 = [(2) –(-1)]2 + [(1) – (-3)]2 = (32 + 42) = (9 + 16) = 25

Hence AB = 5

The distance between the points A and B is 5 units.

Example 2

The coordinates of points P and Q are (4, 3) and (6, 7) respectively. If R is the midpoint of the straight line joining P and Q, what are the coordinates of R?

Solution

The coordinates of the mid point of a line segment with end points as (x1, y1) and (x2, y2)

are given by, x = ½( x1 + x2) and y = ½( y1 + y2)

x1 = 4, x2 = 6, y1 = 3 and y2 = 7

Therefore the coordinate of the mid point R is,

x = ½( x1 + x2) = ½( 4 + 6) = 5 and y = ½( y1 + y2) = = ½( 3 + 7) = 5

## Additional Problems on Straight Lines

Problem 1

The coordinates of points P, Q and R are (2, -1), (4, 3) and (6, 7) respectively. Show that the points P, Q and R are collinear.

Solution

Three points with coordinates (x1, y1), (x2, y2) and (x3, y3) will be collinear if,

x1 (y3 – y2) + x2 (y1 – y3) + x3(y2 – y1) = 0

x1 = 2, x2 = 4, x3 = 6, y1 = -1, y2 = 3 and y3 = 7

x1 (y3 – y2) + x2 (y1 – y3) + x3(y2 – y1) = 2(7 – 3) + 4 (-1 – 7) + 6[(3 – (-1)]

= 2(4) + 4(-8) + 6(4)

= 8 – 32 + 24 = 0

Hence the points P, Q and R collinear.

Problem 2

Prove that the line joining any point on y-axis to any point on x-axis always forms a right triangle.

Solution

Assume that O is the origin, A is any point on y-axis and B be any point on x-axis.

The coordinates of A will be (0, y) and of B will be (x, 0).

Using distance formula,

OA2 = (0 – 0)2 + (y – 0)2 = y2

OB2 = (x – 0)2 + (0 – 0)2 = x2

AB2 = (0 – x)2 + (y – 0)2 = x2 + y2

Therefore, AB2 = OA2 + OB2

Hence the triangle AOB is a right triangle.

## Related Tags

Help with Straight Lines, Study on Straight Lines, Online help on Straight Lines