Mathematics
Grade 8
15 min
Interpret the graph of a linear function: word problems
Interpret the graph of a linear function: word problems
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1
Introduction & Learning Objectives
Learning Objectives
Identify the independent and dependent variables from a linear graph in a word problem context.
Interpret the real-world meaning of any given point (x, y) on a linear function graph.
Calculate the slope of a linear graph and explain its meaning as a rate of change in a word problem.
Identify the y-intercept of a linear graph and explain its meaning as an initial value or starting point.
Use a linear graph to answer specific questions and make predictions related to a real-world scenario.
Explain the real-world meaning of the x-intercept (if applicable) in a given context.
Translate information from a word problem into a graphical representation and vice versa.
Ever wonder how a simple line on a graph can tell a whole story about how much money you save...
2
Key Concepts & Vocabulary
TermDefinitionExample
Linear FunctionA function whose graph is a straight line. It represents a relationship where there is a constant rate of change between two quantities.The relationship between the number of hours worked and the money earned at a constant hourly wage.
Graph of a Linear FunctionA visual representation of a linear function on a coordinate plane, typically showing how one quantity (dependent variable) changes in relation to another (independent variable).A line on a graph where the x-axis is 'Time (hours)' and the y-axis is 'Distance (miles)'.
Independent Variable (x-axis)The variable that changes independently and is typically plotted on the horizontal (x) axis. Its value determines the value of the dependent variable.In a graph showing distance vs....
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Core Formulas
Slope Formula
$$m = \frac{y_2 - y_1}{x_2 - x_1}$$
Use this formula to calculate the slope (m) of a linear function given any two points $(x_1, y_1)$ and $(x_2, y_2)$ on the line. The slope represents the rate of change.
Slope-Intercept Form
$$y = mx + b$$
This equation represents a linear function where 'm' is the slope and 'b' is the y-intercept. It's useful for understanding the initial value and rate of change directly from the equation.
Interpreting a Point (x, y)
A point $(x, y)$ on a graph means 'When the independent variable is $x$, the dependent variable is $y$.'
To understand the real-world meaning of any point on a linear graph, identify what the x-axis and y-axis represent, then state the values in context.
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Challenging
A hot air balloon is descending. Its altitude is recorded at two points in time: after 10 minutes, it is at 1200 feet, and after 30 minutes, it is at 700 feet. What was the balloon's initial altitude when it started its descent (at time = 0 minutes)?
A.1450 feet
B.1200 feet
C.1700 feet
D.1500 feet
Challenging
A graph shows the cost of a monthly gym membership. It's a linear function where the cost depends on the number of personal training sessions attended. The line passes through (0, 30) and (5, 180). A student uses the graph to predict that 50 training sessions would cost $1530. Why might this be an unreasonable prediction?
A.The calculation is wrong; the cost would be much lower.
B.The gym would likely offer a discount for that many sessions, so the rate would change.
C.It is impossible for a person to attend 50 training sessions in one month.
D.Both B and C are valid reasons.
Challenging
A scientist is cooling a liquid. The temperature is graphed as a linear function of time. The initial temperature is 90°C, and it cools at a rate of 4°C per minute. The scientist needs to know when the liquid will reach 10°C. Which steps are needed to solve this, and what is the answer?
A.Set up the equation y = 90x - 4 and solve for x when y=10. The time is 0.15 minutes.
B.Set up the equation y = -4x + 90 and solve for y when x=10. The temperature is 50°C.
C.Set up the equation y = 4x + 90 and solve for x when y=10. The time is -20 minutes.
D.Set up the equation y = -4x + 90 and solve for x when y=10. The time is 20 minutes.
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