Question 37·200 Super-Hard SAT Math Questions·Algebra
Lines and are perpendicular and intersect at the point . Line has equation , where is a nonzero constant.
Which point lies on line ?
When a problem gives perpendicular lines with a parameter (like ), avoid trying to “solve for .” Instead, take the slope directly from the equation of the given line, flip it and negate it to get the perpendicular slope, then write the second line using point-slope form through the intersection point. Finally, plug each answer choice into the line equation; the correct point will satisfy it exactly.
Hints
Find the slope of the perpendicular line
Identify the slope of line from its equation, then use the fact that perpendicular slopes multiply to .
Use point-slope form through the intersection
Write line in the form using and the slope of .
Plug in a choice
Substitute the - and -coordinates from a choice into your equation for line and see whether both sides match.
Desmos Guide
Create a slider for
Enter k=1 and let Desmos create a slider. Move it to a few nonzero values (for example, 1, 2, and -1).
Graph line
Enter the equation y = -(x+1)/k + 4.
Plot the four answer-choice points
Enter each point as a coordinate:
(-1+3k,7)
(-1-3k,1)
(-1+3k,1)
(2k-1,1)
See which point stays on the line
As you move the slider to different nonzero values, look for the point that remains on the graphed line each time. That choice is the correct answer.
Step-by-step Explanation
Use perpendicular slopes
Line has slope . Since and are perpendicular, line has slope (and this requires , which is given).
Write an equation for line
Line passes through , so using point-slope form:
Test the choices using the equation
A point lies on if it makes the equation true.
Check each option by substituting its and into
Identify the point that satisfies the equation
For :
The two sides match, so the point on line is .