In today's video, we're going to run through some of the common straight line equations that you need to be able to recognise on a graph. To quickly recap, the horizontal axis on a graph is called the x-axis, while the vertical one is called the y-axis. And on this graph, we're going from negative 10 to positive 10 on both axes. Now, because this point here is 4 on the x-axis, all of these points we'll also have an x value of 4. And so if we join them all up, we're going to get the line of x equals 4, because x is 4 for all of them, regardless of what the y value is. We can do the same thing to work out the equations for other vertical lines.
So this one is x equals 9, and this over here is x equals negative 2. So basically, whenever you see a vertical line on a graph, its equation is always going to be x equals something. And to find out what that something is, you just look at where it crosses the x-axis. So this one would be x equals negative seven.
The same thing works for horizontal lines, except this time there will be y equals something. So the top line here which crosses the y-axis at 5, would be y equals 5, whereas the one at the bottom, which crosses at negative 3, would be y equals negative 3. If you had a line that went straight along the x-axis, then that would be y equals 0, because it crosses the y-axis at 0, and similarly a vertical line that goes straight along the y-axis would be x equals 0. The last type of line we need to look at are diagonal lines, like this one. If you pick any point along this line, you'll find that the x coordinate is always the same as the y coordinate.
For example here, x is 6, and y is also 6, whereas down here, x and y are both negative 4. Because of this, the equation for this line is y equals x. You could also call it x equals y, but we generally put the y on the left when we write line equations. Now if the diagonal line was instead going the opposite way, the equation becomes y equals minus x.
Because if y is a positive number, like up here where it's a positive 8, the x will be the negative version of that number, so negative 8. And if we pick a point like this one, which has a positive x value of 9, then the y coordinate would be the negative version, so negative 9. Anyway, that's everything for this video. So if it was helpful, then please do give us a like and subscribe, and cheers for watching!