Test 3 Section 8 - #10 (page 546)

First things first: slopes of perpendicular lines are negative reciprocals. You absolutely need to remember that for the test. That is to say, if you have a line with a slope of 3/5, a line perpendicular to it will have a slope of -5/3.

For this particular problem, we're going to have to use the diagram to figure out the slope of line l, and then use the principle above to figure out the slope of n.

If you look closely at l, you have both its x- and y-intercepts. That gives you an easy way to calculate the slope, which we sometimes simply say is equal to "rise/run". Line l has a rise of -1 while it has a run of 3. Slope of line l = -1/3.

That means the slope of line m HAS TO BE the negative reciprocal of -1/3, or 3. That's choice (E).

Note that guesstimate works very well here to check your work. Line m is steep and definitely has a positive slope. Don't get caught picking a choice that doesn't indicate as much.

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