Question 6·Hard·Linear Equations in Two Variables
For line , the table shows three values of and their corresponding values of .
| 18 | 130 |
| 23 | 160 |
| 26 | 178 |
Line is the result of translating line down units in the -plane. What is the -intercept of line ?
When you are given several points on a line, quickly compute the slope using from any two points, then plug one point into to find the -intercept. For vertical translations, remember that only the -values (and thus ) change; the slope stays the same, and shifting down by 5 makes the new intercept . To find an x-intercept on any line, set and solve the resulting one-step equation for . This approach avoids unnecessary graphing and lets you answer translation-and-intercept questions quickly and accurately.
Hints
Use the table to find the slope
Pick any two points from the table and compute the slope of line h using .
Write an equation for line h
Put the slope you found into and use one point from the table to solve for , the -intercept of h.
Adjust for the translation and then find the x-intercept
Translating a line straight down changes only its -values (and -intercept), not its slope. After updating the equation for this vertical shift, set and solve for to get the x-intercept.
Desmos Guide
Use Desmos to confirm the slope of line h
In an expression line, type (160-130)/(23-18) and note the value Desmos gives; this is the slope of line h.
Use Desmos to find the y-intercept of line h
In a new expression line, type 130 - (slope)*18, replacing slope with the value from Step 1 (for example, 130 - 6*18); the result is the value in for line h.
Graph lines h and k
Enter the equation for line h as y = 6x + 22. Then, to represent the line translated down 5 units, enter y = 6x + 17 for line k. You should see two parallel lines, with line k below line h.
Find the x-intercept of line k
Add another expression y = 0 to show the x-axis. Click or tap the intersection point of y = 6x + 17 and y = 0; read off the x-coordinate of this intersection. That x-value is the x-coordinate of the x-intercept of line k.
Step-by-step Explanation
Find the slope of line h
Because h is a line, we can use any two points from the table to find its slope .
Using and :
So line h has slope .
Write the equation of line h
A line with slope has the form .
Substitute and one of the points on h, say :
Since , we get , so .
Thus line h has equation .
Translate the line down 5 units to get line k
Translating a graph down 5 units subtracts 5 from every -value, which changes an equation to ; the slope stays the same.
Here , so for line k the new -intercept is .
Therefore line k has equation .
Find the x-intercept of line k
The x-intercept is where the line crosses the x-axis, so .
For line k, solve .
Subtract 17 from both sides: , so .
Therefore, the x-intercept of line k is .