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# A Couple of Points on Straight Line Graphs...

Here we look at a great way to construct/sketch a straight line graph that requires just two points.

## The two-point gradient and y-intercept method

The equation for a straight line is Where x and y are variables that relate to the x and y axes of a graph.

The gradient (slope) of the line is given by m - this is the number immediately before the x (it's known as the coefficient). If the coefficient is a negative value, the gradient will slope downwards on a graph.

The c term is the y-intercept - the point on the y-axis where the line will cross.

Look at the following equation... If it can be manipulated so that it has the form y = mx + c, it can be graphed. In this case, the line will have a gradient of 3 and will cross the y-axis at y = 4.

The line can be graphed quickly by plotting just two points.

The easiest point to plot is the y-intercept. This occurs when x = 0 so the coordinates will be (0, 4) The y-intercept

Another point on the graph is needed and it can be found using the value for the gradient, 3.

The gradient is usually found by calculating... ...the change in y divided by the change in x for 2 points on a line.

Looking at the equation, this is already done. The value 3 can be represented as . This just means that for every unit increase in a value for x, there will be 3-fold unit increase in the value of y.

Starting with the y-intercept coordinate, move 1 place along the x-axis and 3 places up the y-axis, to get a second coordinate as follows:

The x value will move from 0 to 1 and the y value will move from 4 to 7 (increasing by 3). The new coordinate will be (1, 7). Graph showing two coordinates

Complete by lining up a straight edge to the two coordinates and pencil in the line. This line represents the equation and can be read off to find the corresponding values for the variables x and y. Graph of y = 3x + 4

## The x-intercept

The point where the line crosses the x-axis can also be found from the equation... Set the y value to 0 (the value for y when the value for x lies directly on the x-axis)... Make x the subject... ## Summary

• Find and plot the y-intercept from the equation for a straight line

• Use the coefficient of x to find a second point on the graph

• Line up the points and draw a straight line!

Have a go at plotting the following. You can construct your own graphs or use the Graphing Blank provided below. Graph paper

2x Graphing Blank
.pdf

Here are the solutions

Solutions to Straight-line Graph Problems
.pdf 