Question 134·Medium·Linear Equations in Two Variables
Line passes through the point and is perpendicular to the line in the -plane. What is the equation of line in slope–intercept form?
For line questions involving perpendicular or parallel lines, first rewrite any given equation into slope–intercept form so you can quickly see the slope. Use the fact that perpendicular slopes are negative reciprocals (and parallel slopes are equal), then plug the given point into point-slope form to build the new line and simplify to slope–intercept form, finally matching coefficients and intercept with the answer choices rather than redoing the whole process for each option.
Hints
Find the slope of the given line first
Rewrite in the form so you can see its slope. What do you get for ?
Use perpendicular slope relationship
For two lines to be perpendicular, their slopes must be negative reciprocals. If the slope of one line is , what should the other line's slope be?
Use the point the line passes through
Once you know the slope of line and that it goes through , plug these into point-slope form and then rewrite in slope–intercept form.
Desmos Guide
Graph the original line
In Desmos, enter the rearranged form of the given line: type y = (3/2)x - 2 in one expression line to see its graph and note its slope .
Determine the perpendicular slope
Remember that a perpendicular line must have slope equal to the negative reciprocal of . Use this value (call it ) for the slope of line .
Create a family of candidate lines
In a new expression line, type y = m*x + b using your perpendicular slope value for m, and make b a slider (Desmos will prompt you). This shows all lines with the correct perpendicular slope as you move the slider.
Use the point to find the correct line
Plot the point (-2, 3) in Desmos. Adjust the slider for until the line passes exactly through this point; then read off the resulting equation and match it to the correct answer choice.
Step-by-step Explanation
Rewrite the given line to find its slope
The given line is
Solve for to put it in slope–intercept form.
First, isolate the term:
Now divide everything by :
So the slope of the given line is .
Find the slope of the perpendicular line
If two lines are perpendicular, the product of their slopes is . That means the perpendicular slope is the negative reciprocal of the original slope.
The original slope is , so the perpendicular slope is
So line must have slope .
Use point-slope form with the given point
Line passes through and has slope . Use point-slope form:
Here, and , so
Now expand the right side:
Convert to slope–intercept form and match the choice
Add to both sides to solve for :
Write as and combine like terms:
This matches choice D, so the equation of line is .