Coordinate Plane, CAT Exam 4, Question #24

[clarence.booth]

Hi,

I'm curious about this problem and the slope formula used:

Line A is drawn on a rectangular coordinate plane. If the coordinate pairs (3, 2) and (-1, -2) lie on line A, which of the following coordinate pairs does NOT lie on a line that is perpendicular to line A?

The explanation says the slope of a line can be found using the following equation: slope = (y2 – y1) ÷ (x2 – x1). The problem was set up as such: (2 – (-2)) ÷ (3 – (-1))

My question: Looking at the given coordinates, I would have thought we'd label the coordinates (3,2) and (-1,-2) as (X1,Y1) and (X2, Y2). But instead, the labeling seems reversed in the answer: (x2, y2) and (x1,y1).

Why is this?

Let me know if my question is unclear.

Thanks,

Clarence

[RonPurewal]

it doesn't matter which point is labeled with 1's and which point is labeled with 2's.

if you switch them, then your slope formula will just give you –4/–4 instead of 4/4 (or vice versa). ultimately the value is the same.

(for the same reason, you can also switch the 1's and the 2's in the slope formula itself... as long as you switch both pairs!)

[clarence.booth]

I see it now. Thanks, Ron.

[RonPurewal]

you're welcome.