Question 9·Hard·Linear Functions
For the linear function , the table shows three values of and their corresponding values of .
If , which equation defines ?
For linear-function questions with a table, quickly compute the slope using any two points, then write the function in the form by substituting one point to solve for . After you have the original function, carefully apply any given transformation (like adding or subtracting a constant, or composing with another function) to get the new function, and finally match that simplified expression to the answer choices, paying close attention not to stop at the intermediate function.
Hints
Find the slope of
Pick any two points from the table and compute the slope . Using the last two points keeps the fractions simple.
Write the equation of
Once you know the slope , write in the form and plug in the coordinates of one table point to solve for .
Use the definition of
After you know , replace in with your expression for , then simplify.
Desmos Guide
Recreate the linear function
From the table, we found that the slope of is and that , which leads to . In Desmos, type f(x) = 5x + 20 to graph this function.
Define in terms of
In a new line in Desmos, type h(x) = f(x) - 13. Desmos will automatically show the graph and expression for based on this definition.
Check consistency with the table and choices
Use the Desmos input line or table feature to evaluate at , , and . Then compare the pattern of outputs to what each answer choice would produce (all choices are lines with slope 5 but different intercepts) and see which one matches the values shown by Desmos.
Step-by-step Explanation
Use the table to find the slope of
Because is linear, its graph is a straight line with a constant slope.
Use any two points from the table. The second and third points are convenient:
- Point 1:
- Point 2:
Compute the slope :
So the slope of is . That means has the form for some constant .
Find the full equation for
Now use any point from the table to solve for in .
Use the point :
So
Thus, the linear function is
Express in terms of using
We are told that .
Now substitute the expression we found for :
This gives as a linear expression in that you can simplify.
Simplify and match to the answer choices
Simplify the expression from the previous step:
So the equation that defines is
which corresponds to choice B.