Question 86·Medium·Linear Functions
The function is linear and satisfies and . Which equation defines ?
For linear function questions where you are given two input–output pairs, first convert them into points and quickly compute the slope using . Then, either (1) write and solve for using one point, or (2) in multiple choice, immediately eliminate options with the wrong slope and test a remaining choice by plugging in one of the given points. This minimizes algebra and helps you move quickly while avoiding sign mistakes.
Hints
Turn function values into points
Treat and as points on the graph of a line. What ordered pairs do these give you?
Use the slope formula
Once you have the two points, use the slope formula to find the slope of the line.
Write the line with that slope
After finding the slope, write the equation in the form using that slope, and then plug in one of the points to solve for .
Check against the choices
When you have an equation, compare its slope and intercept to each answer choice, or plug one of the given points into each choice to see which one works.
Desmos Guide
Graph all four answer choices
In Desmos, enter each option on its own line: y=-x-5, y=x+5, y=x-5, and y=-x+5. This will display the four possible lines.
Plot the given points
Add the points (-4,9) and (11,-6) in Desmos (for example, type (-4,9) and (11,-6) on separate lines). These represent the conditions and .
See which line matches both conditions
Look at the graph and see which of the four lines passes exactly through both plotted points. The equation of that line is the correct choice.
Step-by-step Explanation
Interpret the given information as points
Because is linear and you are told and , you can think of these as two points on the graph of :
- Point 1:
- Point 2:
The question is asking for the equation of the line that passes through these two points.
Find the slope of the line
Use the slope formula with the two points and :
So the slope of the line is .
Write a generic equation with the found slope
Use slope-intercept form , where is the slope and is the -intercept.
You found , so the equation has the form
for some value of that you still need to find.
Use one point to solve for the y-intercept
Substitute one of the known points into to find . Use :
So the -intercept is .
Match your equation to the answer choices
You found that the equation of is
Among the choices, this corresponds to choice D.