### Video Transcript

In this video, weβre going to look
at how to find the equation of a straight line in various different forms, given two
pieces of information, the slope of the line and the π¦-intercept. Firstly, a reminder of the
different formats that you may be asked to give the equation of a line in. The first is slope-intercept form,
π¦ equals ππ₯ plus π. The letters π and π each
represent particular properties of the straight line. π represents the slope of the
line, which means, for every one unit you move to the right, the line moves this
many units either up or down, depending on whether π is positive or negative.

π represents the π¦-intercept of
the line, which is the value at which the line cuts the π¦-axis. The second common format for the
equation of a straight line is point-slope form, π¦ minus π¦ one equals π π₯ minus
π₯ one. π represents the slope of the
line, as we saw previously. π₯ one, π¦ one represents the
coordinates of any particular point that lies on this given straight line. Now, in this video, weβre looking
at finding the equation of a line given its slope and its π¦-intercept. And therefore, itβs usual that we
would be using the slope-intercept form in order to do this.

Determine, in slope-intercept
form, the equation of the line which has a slope of eight and a π¦-intercept of
negative four.

So weβre told the way that our
answer should be expressed, slope-intercept form, π¦ equals ππ₯ plus π. So what we need to do is
determine the values of π and π for this question. Weβre actually given the values
of π and π explicitly within the information in the question itself. Weβre told that this line has a
slope of eight; this means that the value of π is eight. Weβre also told that the line
has a π¦-intercept of negative four; this means that the value of π is negative
four. So all we need to do to answer
this question is substitute the values of eight and negative four into the
slope-intercept form of a straight line. Therefore, we have that the
equation of the straight line is π¦ equals eight π₯ minus four.

Find the coordinates of the
point where π¦ equals four π₯ plus 12 intersects the π¦-axis.

So in this question, weβre
given the equation of a straight line in slope-intercept form. And weβre asked for the
coordinates of the point where it intersects the π¦-axis. What this question is checking
then is do we understand slope-intercept form and what the different parts of
the equation represent. Remember that slope-intercept
form is π¦ equals ππ₯ plus π, where π represents the π¦-intercept of the
straight line, which is what weβre looking for here. The π¦-intercept is the point
where the line intersects the π¦-axis. So we can see by comparing the
general form and the specific straight line that we have, the value of π here
is 12.

But the question doesnβt just
ask us for the value of π; it asks us for the coordinates of this point. So the π¦-intercept remember is
a point on the π¦-axis. Weβve just worked out its
π¦-coordinate; itβs this value of 12. To work out the π₯-coordinate,
we just need to remember that at every point on the π¦-axis the π₯-coordinate is
zero. You could perhaps see this more
clearly by picturing what the graph would look like as Iβve done here. So the coordinates of this
point then are going to be zero, 12. And that is our final answer to
this question.

Write the equation represented
by the graph shown. Give your answer in the form π¦
equals ππ₯ plus π.

So we have a diagram of a
straight line. And weβre asked to give its
equation in slope-intercept form, which means we need to work out what each of
these two things are. From looking at the diagram, we
can see that the π¦-intercept is negative four, which means that the value of
π, which is the letter used here to represent the π¦-intercept, must be
negative four. So I can write down the
beginnings of the equation of this straight line; itβs π¦ equals ππ₯ minus
four. Next, we need to find the value
of π, the slope of this line. And in order to do this, I need
the coordinates of two points that lie on the line.

Weβve already had identified
one point, the point with coordinates zero, negative four. Looking at the graph, I can
also see that thereβs a point here that would be convenient to use. This point lies on the π₯-axis
and has the coordinates six, zero. So Iβm going to use these two
points to calculate the slope of the line. So, the slope of the line can
be calculated as a change in π¦ divided by a change in π₯. Or you can think of this as π¦
two minus π¦ one over π₯ two minus π₯ one, if you choose to label the two points
as π₯ one, π¦ one and π₯ two, π¦ two. Iβm just going to look at the
diagram in order to work out the change in π¦ and the change in π₯.

The change in π¦ first of all
then, well that is the vertical length in this triangle. And I can see that it moves
from a π¦-coordinate of negative four to a π¦-coordinate of zero. Therefore, the change in π¦ is
positive four. Now, letβs look at the change
in π₯; this is the horizontal change. So I can see from the diagram
that this moves from a value of zero to a value of six, which gives me a change
in π₯ of positive six. So the slope of this line then
is four over six. But this can be written as a
simplified fraction; itβs two-thirds. Finally then, I just need to
substitute this value of π, the slope of the line, into the equation. So the equation of the line
represented by this graph is π¦ equals two-thirds π₯ minus four.

Find the equation of the line
with π₯-intercept three and π¦-intercept seven and calculate the area of the
triangle on this line and the two coordinate axes.

So this question has two
parts. Weβre first asked to find the
equation of a line and then weβre asked to calculate the area of this
triangle. I think a diagram would be
helpful here in order to visualize the situation. So we have a pair of coordinate
axes. Weβre told that this line has
π₯-intercept three, which means it cuts the π₯-axis at three. Weβre also told the line has
π¦-intercept seven, so it cuts the π¦-axis at seven. By connecting these two points,
I have the line that Iβm looking to find the equation of and I can see the
triangle that Iβm asked to find the area of. Itβs this triangle here.

So letβs start with the first
part of this question, which asks to find the equation of this line. Iβm going to do this using the
slope-intercept form, π¦ equals ππ₯ plus π. I can work out one of these two
values straight away. Remember π represents the
π¦-intercept of the line. And Iβm told in the question
that this is equal to seven. So the equation of the line is
π¦ equals ππ₯ plus seven. I now need to work out the
slope of this line. And in order to do so, I need
the coordinates of two points on the line. Well, I can use the coordinates
of these points, the π₯-intercept and the π¦-intercept.

The slope of the line remember
is calculated as the change in π¦ divided by the change in π₯. So looking at my diagram and
using these two points, Iβm gonna find the change in π¦ first of all. I can see that as I move from
left to right across the diagram, the π¦-coordinate changes from seven to zero,
which is a change of negative seven. Itβs really important that you
consider this change in π¦ as negative seven, not seven. The line is sloping downwards
from left to right, and therefore it has a negative gradient. Now, letβs look at the change
in π₯. I can see that as I move from
left to right across this diagram, the π₯-coordinate changes from zero to three,
which gives me a change in π₯ of positive three.

Now, I can substitute the
change in π¦ and the change in π₯ into my calculation for the slope of this
line. And we have that the slope of
the line is equal to negative seven over three. Finally, in order to complete
the first part of the question and find the equation of the line, I need to
substitute this value for π into the equation. I have then that the equation
of this line is π¦ equals negative seven over three π₯ plus seven. Now, sometimes you may be asked
to give your answer in a slightly different format, for example, a format that
doesnβt involve fractions. So youβd need to multiply the
equation there by three, but as it hasnβt been specified here Iβm going to leave
my answer as it is now. So thatβs the first part of the
question completed.

The second part asked me to
calculate the area of the triangle formed by this line and the two coordinate
axes. Now from the diagram, we can
see that this is a right-angled triangle because the π₯- and π¦-axes meet at a
right angle. To find the area of a
right-angled triangle, we need to multiply the base by the perpendicular height
and then divide by two. So looking at the diagram, I
can see that the base of this triangle is that measurement of three units. The height of the triangle is
seven units. Now, we refer to this as
negative seven when weβre calculating the slope of the line because the
direction was important. For when weβre just looking at
the length of that line in order to calculate an area, weβll take its positive
value of seven. So our calculation for the area
is three multiplied by seven divided by two. And this gives us an answer of
10.5 square units for the area of this triangle.

In summary then, when weβre given
the slope and the π¦-intercept of a straight line, itβs usual to calculate its
equation in slope-intercept form, π¦ equals ππ₯ plus π because the values can be
just substituted directly into this form. We may need to calculate the slope
of the line ourselves given two points on the line by using change in π¦ divided by
change in π₯. It would also be possible to give
the equation of a line in point-slope form, but this would be unnecessarily
complicated if the information weβre given is slope and π¦-intercept as this aligns
so conveniently with the slope-intercept form.