y = mx + c
How to plot straight line graphs
Linear equations
Linear equations are equations that are of the first order. Linear equations are straight-line equations that have simple variable expressions with terms and without exponents. If we come across an equation with $$x$$ or $$y$$, it means we deal with a straight-line equation. If the slope of a line and $$y$$-intercept are given, we use the slope-intercept formula to find the equation of the line.
E 3.4: Interpret and obtain the equation of a straight line graph.
The equation of a straight line with gradient $$m$$ and $$y$$ intercept $$c$$ on the $$y-axis$$ is written in the form,
$$y=mx+c$$.
The slope of the line is known as the gradient and is represented by the value of $$m$$.
The point at which the line crosses the $$y$$ axis is known as $$y$$ intercept and if we look at the general equation of line, $$c$$ is the value of $$y$$ intercept.
Worked examples of plotting gradients
Example 1: A rental company charges a flat fee of $$\$30$$ and an additional $$\$0.25 / \text{mile}$$ to rent a moving van. Construct a linear equation to approximate the cost $$y$$ (in dollars) in terms of $$x$$ that is equal to the number of miles driven. How much will $$75-\text{mile}$$ trip cost?
Step 1: Construct an equation using the values given in the question.
Let the total cost is $$y$$, the flat fee is $$\$30$$, and the additional charges per mile is $$\$0.25 / \text{mile}$$.
If someone travels for $$x\;\text{miles}$$, the additional fee will be $$0.25\times x$$.
Therefore, the equation becomes $$y=0.25x+30$$.
Step 2: Recall the formula for the equation of the straight line.
$$y=mx+c$$
Step 3: Compare the constructed equation with the general equation of the straight line
We get $$m=0.25$$ and $$c=30$$.
Step 4: Find the value of $$y$$ substituting $$x=75$$ in the equation of the line.
$$y=0.25\times 75 + 30$$
Step 5: Solve further
$$y=18.75 + 30$$
Step 6: Add the value and find $$y$$.
$$y=48.75$$
Thus, the cost of a $$75\; \text{mile}$$ trip is $$\$48.75$$.
Step 7: Use the slope and $$y$$-intercept to graph the equation.