Question 29·Medium·Linear Equations in Two Variables
Line passes through and is perpendicular to the line . What is the -intercept of line ?
For perpendicular-line questions, first convert the given line to slope-intercept form to read off its slope quickly. Then take the negative reciprocal of that slope for the perpendicular line, plug the given point into point-slope form to get the new line’s equation, and finally use the definition of the requested intercept (set for an -intercept) to solve efficiently for the needed coordinate.
Hints
Identify the slope of the given line
First, rewrite in the form . What is the coefficient of in that form?
Use the perpendicular slope relationship
If one line has slope , a line perpendicular to it has slope . Apply this to the slope you just found.
Use the point and then find the intercept
Write an equation for the new line using its slope and the point (for example, with point-slope form). Then, to get the -intercept, what value should have, and how can you solve for ?
Desmos Guide
Graph the original line
Type y = (4x - 7)/2 into Desmos to graph the line and visually confirm its positive slope.
Graph the perpendicular line through (2,3)
In a new line, type y - 3 = -1/2 (x - 2) (or y = -1/2x + 4) to graph the line that is perpendicular to the first line and passes through .
Find the x-intercept from the graph
Look at where this perpendicular line crosses the -axis (where ). Click that intersection point and read off the -coordinate and -coordinate; this point is the -intercept, which should match one of the answer choices.
Step-by-step Explanation
Find the slope of the given line
Rewrite in slope-intercept form .
So the slope of this line is .
Use the perpendicular slope
For perpendicular lines, the slopes are negative reciprocals.
- The given line has slope .
- The negative reciprocal is .
So the slope of line is .
Write the equation of line ℓ using the given point
Line passes through and has slope , so use point-slope form:
Now solve for to get slope-intercept form:
This is the equation of line .
Find the x-intercept of line ℓ
The -intercept is where the line crosses the -axis, so .
Set in :
So the -intercept of line is .