Parallel and Perpendicular
Another very common thing to see is that instead of you getting two points, you only get one point, but they tell you that your line is either parallel or perpendicular to other sample line that they tell you. In order to solve this question, you have to know what the deal with parallel and perpendicular lines is. Parallel lines are two lines that kind of look like train tracks. They go in the same direction that ends up meaning that they have the exact same slope. They go over and up the exact same amount. So parallel lines have the same slopes.
Perpendicular lines, on the other hand, are two lines that intersect each other at right angles. Perpendicular lines' slopes are what we call opposite reciprocals. One line is going over a lot, and up a little and one line is going over a little and down a lot. That has to do with the reciprocal part, which essentially means that if you have a fraction (1/4), you flip it (4/1).
So opposite reciprocals are numbers where one is positive (1/4) and one is negative (-4/1), and one is one fraction and the other is the flipped fraction. Don't forget to do both. It's really common to just make it negative or just make it positive, or forget to flip it or flip it but forget to change the sign. It's both.
Finding a Parallel Equation
Finding a Parallel Equation
Now that we know about parallel and perpendicular lines, we can answer questions like this that ask us, 'What is the equation of a line that is parallel to y = 3x + 2 and through the point (-3,6).'
Again, the two things we need to find are m and b, the slope and the y-intercept. We always want to find the slope first, so now I have to go about finding the slope in a slightly different manner than we did earlier. I can no longer use the slope formula, because it only gives us one point, so I have to use what I know about parallel lines.
Luckily, we just learned that parallel lines have the same slope, which means that my slope is the exact same as this one, so I can just take the slope from this line (3) and just steal it and I already know the slope; all my work is done and I have half the answer to my question (y = 3x + b)
The other half that I still need is to find the b and I'm going to do this in the exact same way that we just did. I now know a sample x (-3), a sample y (6), I know what m is (3), all I have to do is find b. I can substitute in what I know (6 = 3 × -3 + b). I can solve the equation for b by doing inverse operations to get the b by itself, and we find that b = 15. Now that we know b and m, we have our answer, which is y = 3x + 15.
Solving a Perpendicular Equation
Similarly, you could be asked to find the equation of the line that is perpendicular to this one through this point. We've got the same equation (y = 3x + 2) and the same point (-3,6), but now instead of it being parallel, it's perpendicular.
I still need to find the m and the b, just like before, and again, I cannot use the slope formula because I only have one point. That means we use what we know about perpendicular lines and how their slopes relate. I know that the slope of y = 3x + 2 is 3, but because it's perpendicular, I can't simply steal that.
I have to first take the opposite reciprocal of it. The opposite part means that it changes from positive (3) to negative (-3). The reciprocal part means that it changes from -3/1 to -1/3. And now, we have the slope (-1/3) of our line, which means that we have half our answer (y = -(1/3)x + b) and all that's left to find is b.
We find b in the exact same way we found b in every other problem in this video. We substitute in our sample x (-3), our sample y (6), what we know m is (-1/3) and we solve the equation for b (6 = -(1/3)(-3) + b). I multiply what I can and I undo to get b by itself, and this time we find that b = 5. This means my solution is y = -(1/3)x + 5.