Question 48·Hard·Linear Equations in Two Variables
In the -plane, line passes through the point and is perpendicular to the line that passes through and . Which of the following is an equation of line ?
For perpendicular-line questions, quickly compute the slope of the given line using , then take the negative reciprocal to get the perpendicular slope. On multiple-choice problems, either use point-slope form with the given point and then convert to match the options, or more efficiently, convert each option to slope-intercept form to compare slopes and test whether satisfies the equation; the only line with the correct perpendicular slope that also contains the point is the answer.
Hints
Start with the given two points
Use the slope formula with the points and to find the slope of the given line.
Use the relationship for perpendicular slopes
Once you know the slope of the given line, remember that the slope of a perpendicular line is the negative reciprocal (flip the fraction and change the sign).
Use the point on line p
Line must pass through , so use point-slope form with the perpendicular slope and this point to write an equation.
Connect to the answer choices
Rewrite your equation into the form (no fractions) and see which option has the same coefficients and constant term.
Desmos Guide
Use Desmos to find the slope of the given line
In an expression line, type (-1 - 3) / (4 - (-2)) to compute the slope of the line through and . Note the value shown (it should be a negative fraction).
Determine the perpendicular slope
Take the negative reciprocal of the value from step 1 (flip the fraction and change the sign). For example, if Desmos shows , then the perpendicular slope is .
Graph the answer choices
In separate expression lines, type each option exactly as written: 2x + 3y = 23, 3x - 2y = 23, 2x + 3y = -23, and 3x + 2y = 23. Desmos will graph all four lines.
Identify the correct line using slope and point
Check which graphed line has the perpendicular slope from step 2 and passes through the point (you can click on the graph near to see whether the point lies on that line). The line that meets both conditions is the correct choice.
Step-by-step Explanation
Find the slope of the given line
Use the slope formula with the points and :
So, the slope of the line through and is .
Find the slope of a perpendicular line
Perpendicular lines have slopes that are negative reciprocals of each other.
- The slope of the given line is .
- Flip the fraction and change the sign to get the perpendicular slope:
Write an equation using the perpendicular slope and the point
Line passes through and has slope .
Use point-slope form, , with and :
Now solve for to get slope-intercept form:
Convert to standard form and match the choice
Start from the slope-intercept form you found and convert it to the standard form used in the answer choices:
So, an equation of line is , which corresponds to choice B.