Question 116·Medium·Linear Equations in Two Variables
The table lists several pairs that lie on the same line. Which equation could represent the relationship between and ?
| 1 | 17 |
| 4 | 11 |
| 10 | -1 |
For line-from-table questions, first decide if the relationship is increasing or decreasing to determine the sign of the slope, then quickly compute the exact slope using two points with . Eliminate any choices with the wrong slope, then either plug a point into to find the intercept or directly substitute a table point into the remaining answer choices to see which one produces the correct ; this is much faster and less error-prone than trying to work with all three points at once.
Hints
Look at how changes as changes
Compare the -values as goes from to and from to . Is going up or down? That tells you whether the slope is positive or negative.
Find the slope from two points
Use any two points from the table and apply the slope formula . This will help you quickly rule out some choices based on their slopes.
Use with a point from the table
Once you know the slope , plug in and from one of the table points into to solve for , the -intercept.
Check the answer choices using substitution
After you find and , compare with each equation option, or plug in one of the table -values into each choice to see which gives the correct .
Desmos Guide
Enter the table of points
Create a table in Desmos and enter the three points from the problem: , , and . You should see these three points plotted.
Graph each answer choice
On separate lines, type each option exactly as given: y = -2x + 19, y = 2x + 19, y = -2x - 19, and y = -1/2x + 19. Desmos will draw four different lines.
See which line passes through all the points
Look at the graph and identify which of the four lines goes through all three plotted points. The corresponding equation is the correct choice.
Optional: Verify with a table for each line
For extra confirmation, click the gear icon next to each equation and add a table. Check the -values in each line’s table for , , and , and see which one matches the original table exactly.
Step-by-step Explanation
Use the table to determine if is increasing or decreasing
Look at how changes as increases:
- From to , increases by , while changes from to , a decrease of .
- From to , increases by , while changes from to , a decrease of .
So as increases, decreases, meaning the line must have a negative slope.
Calculate the slope of the line
Use the slope formula with any two points, for example and :
So the slope of the line is .
Use slope-intercept form to find the -intercept
The slope-intercept form of a line is , where is the slope and is the -intercept.
We know , and we can use a point from the table, say , to find :
Solve this equation for .
Solve for and write the equation, then match the choice
Continue solving:
So the line has slope and -intercept , which means its equation is . Among the answer choices, this matches choice A.