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# Graphs

Graphs are visual representations of equations that can help us understand the relationship between two variables.

## Using graphs for algebraic relationships

You can use the coordinate plane, which you can see below, to graph any algebraic relationship. The coordinate plane comprises a horizontal line (x-axis) and a vertical line (y-axis), and it is divided into four quadrants named using roman numbers (I, II, III and IV).

The different points in a graph have coordinates written as ordered pairs (pairs of numbers within parentheses separated by a comma). The first number in an ordered pair (x, y) represents the value of x, and the second one represents the value of y for a given point. For example, the middle point where the x and y axes meet is called the origin, and its coordinates are (0, 0).

Graphs help us analyse the behaviour of variables and can be used to make inferences about them and facilitate data interpretation.

Coordinate plane

### Plotting vs sketching

When making graphs, you can either do it by plotting or sketching. For plotting, you normally use graph paper and make a table of values for x and y coordinates and plot them as accurately as possible.

If you have the equation y = x, you can plot its graph like this:

Plotting a graph

 x y -1 -1 0 0 1 1 2 2

When sketching, you do not need to be as accurate. You need to draw the x and y axes and sketch the general shape of the curve, including the points where it intersects the x and y axes. In the case of a line graph, you only need a couple of points to draw the line crossing those two points. When you sketch the line graph for y = x, you only need one more point, as you know that the line crosses the origin (0, 0).

Sketching a graph

## What are the different types of graphs?

Depending on the type of function that you are graphing, you will obtain different characteristic shapes for their curve. The main types of graphs are described below.

### Linear graphs

Linear graphs are a straight line. They represent the graph of functions where the highest exponent in its equation is 1.

• Slopes and intercepts

In linear graphs, the slope is the line's rate of change in the vertical direction. The slope can be shallow or steep, depending on its value. The bigger the value of the slope, the steeper the line will be, and the smaller the value of the slope, the shallower the line will be. Also, you will need to remember that the slope of a horizontal line is zero, and the slope of a vertical line is undefined.

Any linear equation can be represented in the slope-intercept form, like this:

y = m x + b

x = independent variable

y = dependent variable

m = slope (how steep the line is)

b = y-intercept (y coordinate for the point where the line crosses the y axis)

The slope can be calculated using the formula:

m =

m = =

where and are any two points on the line.

If you don't have the equation of the line graph, but you can identify two points on the line A = (2, 2) and B = (5, 5).

Calculating the slope

Then you can calculate the slope as follows:

m = 1

It doesn't matter which one you choose as point 1 and point 2. The resulting slope will be the same.

If you have the linear equation y = mx + b, you don't need to calculate the slope. You can identify in the equation the value of m, which will be the slope of the line. Likewise, the value of b will be the y-intercept.

For the equation y = 2x + 3, m = 2 and b = 3.

If the function we want to graph is quadratic, generically represented as , then the shape of y = f (x) will be a parabola.

If the coefficient of ( ) is positive, then the parabola will be the right way up.

If the coefficient of ( ) is negative, then the parabola will be upside down.

Besides identifying if the parabola will be the right way up or upside down, to sketch a quadratic graph you need to proceed as follows:

• Substitute x = 0 in the function , to obtain the y-coordinate where the parabola crosses the y-axis, which is equal to c.

• Make the function equal to zero, and find the roots of the function f (x). The roots will be the x-coordinates where the parabola crosses the x-axis. You can find the roots by factoring, completing the square, or using the quadratic formula.

• Find the turning point of the parabola (minimum or maximum), either by completing the square or using symmetry.

• If you complete the square, then the turning point will be (-p, q) if .

• If you use symmetry, the x-coordinate of the turning point will be in the middle of the two roots found in the previous step (add them together then divide by 2). After this, you need to substitute the resulting value of x into the original function to find the y-coordinate of the turning point.

• Sketch the graph.

Sketch the graph of , and find the coordinates of its turning point.

• The coefficient of ( ) is positive, therefore the parabola will be the right way up, and it will have a minimum point.
• When x = 0, y = 2, therefore the parabola crosses the y-axis in the point (0, 2)
• Find the roots of the function by factoring

The roots are and

• Using symmetry to find the turning point:

Substitute x in the original equation

The minimum point is

• Now you can sketch the graph:

### Cubic graphs

If the function that you are graphing is cubic , generically represented as , then the shape of y = f (x) is shown below if the coefficient of ( ) is positive.

Cubic graph with a positive coefficient

If the coefficient of ( ) is negative, then the shape will be like this:

Cubic graph with a negative coefficient

To sketch the graph of cubic functions, you need to find the roots of the function.

Sketch the curve for showing the points where they cross the coordinate axes.

• When y = 0,

The roots are , , and

Therefore, the curve crosses the x-axis at (-1, 0), (-2, 0) and (-3, 0)

• When x = 0,

The curve crosses the y-axis at (0, 6)

• Sketch the graph:

Sketching a cubic graph

### Quartic graphs

If the function you are graphing is quartic, generically represented as , then the shape of y = f (x) can have different forms depending on its roots. One of the possible shapes, if the coefficient of ( ) is positive, is shown below.

Quartic graph with positive coefficient

If the coefficient of ( ) is negative, its curve can take the following shape:

Quartic graph with negative coefficient

Again, to sketch the graph of quartic functions, you need to find the roots of the function.

Sketch the curve for showing the points where they cross the coordinate axes.

• When y = 0,

The roots are x = 0, x = 1, x = -3 and x = 2

Therefore, the curve crosses the x-axis at (0, 0), (1, 0), (-3, 0), and (2, 0)

• When x = 0, y = 0

The curve crosses the y-axis at (0, 0)

• Sketch the graph:

Sketching a quartic graph

### Modulus function graphs

The modulus function, also known as the absolute value function, is generically represented. The modulus of a number x will be the same number but positive. The typical shape of a modulus function is shown below.

Modulus function graph

If you have an expression inside the modulus function, calculate the value inside, then find the positive version of the result.

If you have the function find

To sketch the graph of the modulus function , you need to sketch , and reflect the portion of the line that goes below the x-axis into the x-axis.

Sketch the graph for showing the points where they cross the coordinate axes.

Ignoring the modulus, you need to sketch the graph of

• When y = 0, x = 1

The line crosses the x-axis at (1, 0)

• When x = 0, y = -1

The line crosses the y-axis at (0, -1)

• Sketch the graph for :

Sketching a modulus function graph

• For the negative values of y, reflect in the x-axis. In this case, (0, -1) becomes (0, 1)

Sketching a modulus function graph

### Reciprocal graphs

Reciprocal functions are generally represented as , and . To sketch this type of graph, you need to consider its asymptotes. An asymptote is a line that the curve gets very close to, but it never touches it. The graph of reciprocal functions has asymptotes at x = 0 and y = 0. The shape of a reciprocal function where , , is shown below.

Reciprocal graph with a positive coefficient

The shape of a reciprocal function where , , is as follows.

Reciprocal graph squared

Sketch the graph for

Sketching a reciprocal graph

### Circle graphs

Another important type of graph that you will find in Coordinate Geometry is circle graphs. A circle is a set of points that are at the same distance from a fixed point called the centre. The equation of a circle with centre (0, 0) and radius r is . If the centre is (a, b), then the equation changes to .

Circle graph

Write down the equation of the circle with centre (6, 5) and radius 3, then sketch its graph.

Sketching a circle graph

## Graphs - key takeaways

• Graphs are visual representations of equations that can help us understand the relationship between two variables.

• When sketching, you do not need to be as accurate as when plotting; you need to draw the x and y axes and sketch the general shape of the curve, including the points where it intersects the x and y axes.

• The slope of a line and the y-intercept can be used to help graph a linear equation.

• To sketch quadratic, cubic and quartic equations the roots of the function must be identified, as well as the point where the curve crosses the y-axis.

• To sketch the graph of the modulus function , you need to sketch , then reflect the portion of the line that goes below the x-axis into the x-axis.

• In reciprocal function graphs, an asymptote is a line that the curve gets very close to, but it never touches it.

• A circle is a set of points that are at the same distance from a fixed point called the centre.

Graphs are visual representations of equations that can be used to help understand the relationship between two variables.

The slope of a line graph can be calculated using the formula m = (y2 - y1)/(x2 - x1) using any two points (x1, y1) and (x2, y2) on the line. From the linear equation y = mx + b, you can identify the slope as the value of m, and the value of b will be the y-intercept.

• Identify if the parabola will be the right way up (coefficient of x² is positive), or upside down (coefficient of x²  is negative).
• Substitute x = 0 in the function f(x) = ax² + bx + c, to obtain the y-coordinate where the parabola crosses the y-axis, which is equal to c.
• Make the function  f(x) = ax² + bx + c equal to zero, and find the roots of the function f(x). The roots will be the x-coordinates where the parabola crosses the x-axis. You can find the roots by factoring, completing the square or using the quadratic formula.
• Find the turning point of the parabola (minimum or maximum), either by completing the square or using symmetry.
• If you complete the square, then the turning point will be (-p, q) if f(x) = a(x + p)² + q .
• If you use symmetry, the x-coordinate of the turning point will be in the middle of the two roots found in the previous step (add them together then divide by 2). After this, you need to substitute the resulting value of x into the original function to find the y-coordinate of the turning point.
• Sketch the graph.

If you are graphing a line with a fractional slope, for example, y = (1/3)x + 1. First, identify the y-intercept b = 1. This means that the line crosses the y-axis at (0, 1). Draw the point (0, 1) on the coordinate plane, and from there you can use the slope m = 1/3, to identify a second point, by going up 1 and across 3 times, remembering that the slope m = rise/run. If required, identify a couple of more points. Now draw the line crossing all the points identified.

## Final Graphs Quiz

Question

What are graphs?

Graphs are visual representations of equations that can be used to help understand the relationship between two variables.

Show question

Question

What is the coordinate plane?

The coordinate plane comprises a horizontal line ($$x$$-axis) and a vertical line ($$y$$-axis), and it can be used to graph any algebraic relationship.

Show question

Question

What is the origin?

The origin is the middle point where the $$x$$ and $$y$$ axes meet.

Show question

Question

How many quadrants are there in a coordinate plane?

The coordinate plane is divided into four quadrants named using roman numbers (I, II, III and IV).

Show question

Question

What are ordered pairs?

Ordered pairs are pairs of numbers within parentheses separated by a comma. The coordinates of the points in a graph are written as ordered pairs $$(x, y)$$.

Show question

Question

What is the difference between plotting and sketching?

When plotting, you normally use graph paper, and make a table of values for $$x$$ and $$y$$ coordinates, and plot them as accurately as possible. With sketching, you do not need to be as accurate, you just need to draw the $$x$$ and $$y$$ axes and sketch the general shape of the curve, also including the points where it intersects the $$x$$ and $$y$$ axes.

Show question

Question

How many points do you need to plot to draw a line graph?

At least 2.

Show question

Question

What is the slope-intercept form of a linear equation?

$$y=mx+b$$.

Show question

Question

What is the slope of a line?

Slope is the line’s rate of change in the vertical direction. The slope can be shallow or steep depending on its value.

Show question

Question

What is the shape of a quadratic graph?

A parabola

Show question

Question

How do you find the turning point of a parabola?

Find the turning point of the parabola (minimum or maximum), either by completing the square or using symmetry.

• If you complete the square, then the turning point will be $$(-p, q)$$ if $$f(x) = a(x+p)^2+q$$.
• If you use symmetry, the $$x$$-coordinate of the turning point will be in the middle of the two roots of the function (add them together then divide by $$2$$). After this, substitute the resulting value of $$x$$ into the original function to find the $$y$$-coordinate of the turning point.

Show question

Question

How do you sketch cubic and quartic graphs?

To sketch cubic and quartic graphs, find the roots of the function and the point where the curve crosses the $$y$$-axis, then sketch the graph.

Show question

Question

What is the modulus of a number?

The same number, but positive.

Show question

Question

If $$f(x) = |x-5|+2$$ find $$f(-1)$$.

$$8$$.

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Question

In reciprocal functions, what are asymptotes?

An asymptote is a line that the curve gets very close to, but it never touches it.

Show question

Question

What is the equation of the circle with centre $$(3, 4)$$ and radius $$2$$?

$(x-3)^2+(y-4)^2 = 4.$

Show question

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