How do you do Test 9 section 3 number 20?

This question is about perpendicular lines, so the first thing to remember is that perpendicular lines have negative reciprocal slopes. The graph of f, shown, has a slope of -2. That means the slope of g, not shown, must have a slope of \frac{1}{2}.

The second thing to quickly note is that this question is using function notation and asking for g(0). That’s the same as asking for the y-intercept; it’s just the question 20 way to ask it.

We know that g passes through (1, 3), and we know it has a slope of \frac{1}{2}, so all we really need to do is count leftwards. (1, 3) is only 1 unit away from the y-axis, and when you have a slope of \frac{1}{2}, you travel 0.5 units down when you travel 1 unit left. Therefore, the y-intercept—aka g(0)—is 2.5.

If you like algebra better, you can plug the slope and the point you know into slope-intercept form (y=mx+b) and solve for b:


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