00:01
Okay, so we're given the monthly electricity cost based on the number of kilowatt hours used, and we have to find the formula c of x for x greater than 500 kilowatt hours.
00:15
Okay, so basically we've got to find the equation for this line here.
00:24
This red part is the part we actually care about because this is 500 onwards.
00:30
Okay, so what type of line is this? it's a straight line.
00:37
So our first step is figure out what kind of line it is.
00:39
It is indeed linear.
00:41
So that means our formula is going to look something like y equals mx plus b.
00:46
But of course we're given it should be c of x, so it's just going to be c of x equals mx plus b.
00:54
Okay, step two, you've got to figure out what these variables are.
00:57
So it looks like b is going to be our y intercept, of course, basically a starting value.
01:08
And m is going to be our slope, or how much everything, how much things are changing.
01:17
Okay, so, okay, so how do we figure out the slope? because from this graph, because this line goes from here to here, we can't really see the y intercept.
01:29
We can kind of guess that it would go somewhere down there, but we can't see it, so we're going to have to find it mathematically.
01:38
So how do we find the slope of this line? we're going to need two points on the line.
01:43
So for this one, let's do 500 and 100, and then 800, and 250.
01:52
Okay, so now we can figure out our slope, and that is always equal to y2 minus y1 over x2.
02:03
Minus x1.
02:05
So rise, which is the y value, up and down, over the run, which is the x value.
02:11
Okay.
02:13
So if our points are 500, 100, and 800, 250, when we plug this in, we're just going to get 250, 250 minus 100, the y values over the x values, 800 minus 500.
02:37
So we get 150 over 300.
02:44
We can cancel this.
02:45
15 over 30 is one half...