### Video Transcript

Angles of Intersecting Lines in a
Circle

In this video, we will learn how to
find the measures of angles resulting from the intersection of two chords, two
secants, two tangents, or a tangent and a secant in a circle. To do this, letβs start with the
case of two chords intersecting inside of a circle. Here, weβve drawn the chord π΄π΅
intersecting the chord πΆπ·. Weβll call the point of
intersection πΈ.

We want to find an expression for
one of the angles between these two chords. Letβs try and find an expression
for the measure of the angle π΅πΈπΆ. To do this, we start by joining the
points π΄ and πΆ, giving us a triangle π΄πΈπΆ. The first thing we note is angles
π΄πΈπΆ and π΅πΈπΆ make up a straight line. So the sum of their measures is 180
degrees.

Next, we also have that π΄πΈπΆ is a
triangle. So the sum of the measures of the
internal angles of this triangle will be 180 degrees. Both of the expressions on the
left-hand side of the equation are equal to 180 degrees. So the left-hand side of both
equations must be equal. And if we remove the measure of
angle π΄πΈπΆ from both sides of this equation, we have the measure of angle π΅πΈπΆ
must be equal to the measure of angle π΄πΆπΈ plus the measure of angle πΈπ΄πΆ.

Another way of thinking about this
is both sides of the equation add to the measure of angle π΄πΈπΆ to make 180
degrees. We can rewrite this equation even
further. First, letβs take a look at angle
π΄πΆπΈ. We can see that angle π΄πΆπΈ is
exactly the same as angle π΄πΆπ·. Itβs on the circumference of our
circle. In particular, this angle is
subtended by the minor arc from π΄ to π·. And we recall, whenever this
happens, this means the measure of the angle will be one-half the measure of the
arc. The measure of angle π΄πΆπΈ is
one-half the measure of the arc π΄π·. We can then do exactly the same for
our other angle, angle πΈπ΄πΆ. This time, the minor arc from π΅ to
πΆ subtends this angle. Therefore, the measure of angle
πΈπ΄πΆ is equal to one-half the measure of the arc π΅πΆ.

We can substitute both of these
expressions for the angles into our equation. This gives us the measure of angle
π΅πΈπΆ is one-half the measure of arc π΄π· plus one-half the measure of arc
π΅πΆ. We can then take out a factor of
one-half to get the following equation. The measure of angle π΅πΈπΆ is
one-half the sum of the measure of arc π΄π· and the measure of arc π΅πΆ.

Another way of thinking about this
is weβre taking the average of the measures of the two arcs opposite our angle in
the circle. And in exactly the same way, we can
find an expression for one of the other angles at point πΈ. In exactly the same way, the
measure of the angle will be one-half the sum of the measures of the arcs opposite
the angle. The measure of angle π΄πΈπΆ is
equal to one-half times the measure of arc π΄πΆ plus the measure of arc π΅π·.

We can write this result formally
as follows. If two chords π΄π΅ and πΆπ· in a
circle meet at a point πΈ, then the measure of either angle between the two chords
is half the sum of the measures of the arcs opposite the angle, giving us the
following two formulas. The measure of angle π΅πΈπΆ is
one-half the measure of arc π΄π· plus the measure of arc π΅πΆ. And the measure of angle π΄πΈπΆ is
one-half the measure of arc π΄πΆ plus the measure of arc π΅π·. Letβs see an example of applying
this result to find the measure of an angle between two chords in a circle.

Find π₯.

In this question, weβre asked to
find the value of π₯. And we can see that π₯ is the angle
between two chords in our circle. Thatβs the chord π΄π΅ and the chord
πΆπ·. We can recall the following
fact. The angle between two chords in a
circle is one-half the sum of the measures of the arcs opposite the angle. And in our diagram, weβre given the
measures of both of the arcs opposite our angle π₯. Thatβs the arc π΄πΆ, which has
measure 73 degrees, and the arc π·π΅, which has measure 133 degrees. So by applying this result, we must
have that π₯ is equal to one-half times 73 degrees plus 133 degrees. We can then evaluate this
expression. 73 plus 133 is 206, and then
one-half of this is 103 degrees.

Therefore, we were able to show
that if π₯ is the angle shown in the diagram, π₯ is equal to 103 degrees.

We can follow a very similar method
to our last proof to help us find an angle between two secant lines which intersect
outside of a circle, where we remember a secant line is the line extension of a
chord. For example, letβs consider the
following diagram, which has the secant line π΄π΅ and the secant line πΆπ·, which
intersect at point πΈ. We want to find an expression for
the measure of angle π΅πΈπ·. To do this, weβll once again create
a triangle. This time, weβll create a triangle
by connecting the points π΄ and π· with a line.

We can follow the exact same method
we did in the last proof to find an expression for the measure of angle π΅πΈπ·. First, angle πΆπ·π΄ and angle
π΄π·πΈ make up a straight line, so their sum is 180 degrees. And we also know the sum of the
measures of the internal angles of the triangle π΄π·πΈ will also add to 180
degrees. So the measure of angle π΄πΈπ· plus
the measure of angle π·π΄πΈ plus the measure of angle π΄π·πΈ is equal to 180
degrees. Both of these expressions are equal
to 180 degrees. So we can equate the left-hand side
of both equations. And we can also note that the
left-hand side of the equation both have a term measure of angle π΄π·πΈ, so we can
remove this. This gives us the measure of angle
π΄πΈπ· plus the measure of angle π·π΄πΈ is equal to the measure of angle πΆπ·π΄.

One way of seeing this is that both
sides of our equation add to the measure of angle π΄π·πΈ to give us a value of 180
degrees. We want to find an expression for
the measure of angle π΄πΈπ·. So weβll subtract the measure of
angle π·π΄πΈ from both sides of the equation. This gives us that the measure of
angle π΄πΈπ· is the measure of angle πΆπ·π΄ minus the measure of angle π·π΄πΈ.

Finally, we can find expressions
for both of these angles since both of these angles are subtended by arcs in our
circle. First, angle π·π΄πΈ is subtended by
the arc from π΅ to π·. Second, angle πΆπ·π΄ is subtended
by the arc from π΄ to πΆ. And we recall that inscribed angles
are one-half the measure of the arcs that they are subtended by. Therefore, the measure of angle
πΆπ·π΄ is one-half the measure of the arc from π΄ to πΆ and the measure of angle
π·π΄πΈ is one-half the measure of the arc from π΅ to π·. And the difference between these
two values is the measure of angle π΄πΈπ·.

Finally, we can take out the factor
of one-half to get the measure of angle π΄πΈπ· is one-half the measure of arc π΄πΆ
minus the measure of arc π΅π·. We can write this more formally as
follows. If π΄π΅ and πΆπ· are secants which
intersect at a point πΈ outside of our circle, then the measure of the angle between
the two secants is one-half the positive difference between the measures of both
arcs intercepted by the sides of the angle. In other words, the measure of
angle π΄πΈπ· is one-half the positive difference of the measure of arc π΄πΆ and the
measure of arc π΅π·. Letβs see an example of how we can
use this property to determine the angle between two secants which intersect outside
of a circle.

Find π₯.

In this question, weβre asked to
find the value of π₯. And we can see in our diagram that
π₯ is the angle between two secant lines which intersect outside of our circle. And we can find the measure of π₯
by recalling the following fact. The angle between two secant lines
in a circle which intersect outside of a circle is one-half the positive difference
of the measures of the arcs intercepted by the sides of the angle.

To apply this property, letβs do
this step by step. First, letβs mark the sides of the
angle of π₯. We can see that π₯ is the angle
between the lines π΄πΆ and π΄πΈ. So the two sides of our angle are
the line segment π΄πΆ and the line segment π΄πΈ. Next, we need to find the measures
of the arcs intercepted by the two sides of our angle. The first side of our angle
intersects the circle at the point π΅, and the second side of our angle intersects
the circle at the point π·. So one of the arcs weβre going to
use is the arc from π΅ to π·. Similarly, the first side of our
angle intercepts the circle at the point πΆ, and the second side of our angle
intercepts the circle at the point πΈ. So the other arc weβre interested
in is the arc from πΆ to πΈ.

Finally, the measure of our angle
will be one-half the positive difference between the measures of these two arcs. And since the arc from πΆ to πΈ is
bigger than the arc from π΅ to π·, this gives us the following result. π₯ will be equal to one-half
multiplied by the measure of arc πΆπΈ minus the measure of arc π΅π·. And weβre given both of these
values in the diagram. The measure of arc πΆπΈ is 132
degrees, and the measure of arc π΅π· is 36 degrees. So we substitute these values into
our formula. We get that π₯ is equal to one-half
multiplied by 132 degrees minus 36 degrees. And we can then evaluate this
expression. 132 minus 36 is equal to 96. And if we multiply this by
one-half, we get 48. Therefore, π₯ is equal to 48
degrees.

Therefore, we were able to find the
value of π₯ in the given diagram. It was one-half the difference
between the measure of arc πΆπΈ and the measure of arc π΅π·, which was 48
degrees.

Letβs now consider how we would
find the angle between two tangent lines to a circle which intersect at a point
outside of the circle. For example, letβs consider the
following tangent lines which intersect at the point πΆ. We want to determine the measure of
angle π΄πΆπ΅. If we call the center of our circle
π, then ππ΄πΆπ΅ is a quadrilateral. And the internal angles of a
quadrilateral add to 360 degrees.

So the measure of angle π plus the
measure of angle π΄ plus the measure of angle πΆ plus the measure of angle π΅ is
equal to 360 degrees. Since π΄ and π΅ are the points of
tangency and π is the center of our circle, the angle at π΄ and the angle at π΅
will be right angles. So these are both equal to 90
degrees. So we can subtract 180 degrees from
both sides of the equation to get the measure of angle π plus the measure of angle
πΆ is equal to 180 degrees. And we can see in our diagram that
angle π is the central angle of a circle and is subtended by the arc from π΄ to
πΆ. And we know the measure of an arc
is equal to the measure of its central angle. So in this case, the measure of
angle π is equal to the measure of the arc from π΄ to π΅.

We can then substitute this into
our equation and then rearrange to find an expression for the measure of angle
πΆ. We get that the measure of angle πΆ
is equal to 180 degrees minus the measure of the arc from π΄ to π΅. And we can also write the result
weβve just proven formally as follows. If two tangents to a circle at
points π΄ and π΅ intersect at a point πΆ, then the measure of the angle between the
tangents is 180 degrees minus the measure of the arc between the two points of
tangency. The measure of angle πΆ is equal to
180 degrees minus the measure of the arc from π΄ to π΅.

Letβs now see an example where we
use this property to determine the angle between two tangent lines of a circle which
intersect at a point outside of the circle.

Find π₯.

In this question, weβre asked to
find the value of π₯. And we can see that π₯ is the angle
between two tangent lines to our circle. Thatβs the line from π΄ to πΆ and
the line from π΄ to π΅. They just touch the circle at a
single point, so these are tangent lines. And we can find the value of π₯ by
recalling the following property for the angle between two tangent lines which
intersect at a point outside of our circle.

We recall the angle between two
tangent lines which intersect at a point is 180 degrees minus the measure of the arc
between the two points of tangency. In our diagram, the points of
tangency are the points π΅ and πΆ. And the arc between π΅ and πΆ will
be the minor arc shown. And we know the measure of this
arc; its measure is 151 degrees. Then, our property tells us that
the value of π₯ is equal to 180 degrees minus the measure of arc π΅πΆ. So we can substitute the measure of
arc π΅πΆ, being 151 degrees, to get π₯ is equal to 180 degrees minus 151 degrees,
which we can calculate is 29 degrees.

Therefore, by using the fact that
the angle between two tangent lines which intersect at a point outside of a circle
is 180 degrees minus the measure of the arc between the two points of tangency, we
were able to show that π₯ is equal to 29 degrees.

Finally, letβs try and find the
angle between a tangent line and a secant line which intersect outside of a
circle. In this diagram, the tangent line
is π΄π· and the secant line is πΆπ΅. And we want to find the measure of
the angle π΄π·π΅. Weβll do this by using a very
similar method to the last three proofs. Weβll start by connecting π΄ and π΅
to construct a triangle π΄π΅π·. We see that angle πΆπ΅π΄ and angle
π΄π΅π· are on a straight line, so their measures add to 180 degrees. So we have the measure of angle
πΆπ΅π΄ plus the measure of angle π΄π΅π· is equal to 180 degrees.

We then also have that the sum of
the measures of the internal angles in a triangle add to 180 degrees. So we have the measure of angle
π΅π·π΄ plus the measure of angle π΅π΄π· plus the measure of angle π΄π΅π· is equal to
180 degrees. And now we have two different
expressions which when added to the measure of angle π΄π΅π· is equal to 180
degrees. So these two expressions must be
equal. The measure of angle πΆπ΅π΄ is
equal to the measure of angle π΅π·π΄ added to the measure of angle π΅π΄π·.

We can subtract the measure of
angle π΅π΄π· from both sides to find an expression for the measure of angle
π΅π·π΄. We have the measure of angle π΅π·π΄
is equal to the measure of angle π΅π΄π· minus the measure of angle πΆπ΅π΄. We can find an expression for the
measure of angle π΅π΄π· by first adding the following two radii to our diagram. And then weβll use the fact that
the measure of the internal angles of quadrilateral ππ΄π·π΅ add to 360 degrees. Since π΄ is a point of tangency for
our tangent line, angle ππ΄π· is a right angle. So the sum of the internal angles
of this quadrilateral β the measure of angle π΄ππ΅ plus 90 degrees plus the measure
of angle π΅π·π΄ plus the measure of angle π·π΅π β is equal to 360 degrees.

We know the measure of the central
angle π΄ππ΅ will be equal to the measure of the arc π΄π΅. So we can substitute this into our
expression to give us the following. And by considering the internal
angles of triangle π΄π΅π·, the internal angles sum to 180 degrees. So the measure of angle π΄π΅π· is
180 degrees minus the sum of the other two angles, the measure of angle π΅π΄π· and
the measure of angle π΅π·π΄. Finally, since ππ΄ and ππ΅ are
radii, this means ππ΄π΅ is an isosceles triangle. Therefore, the measure of angle
ππ΄π΅ and the measure of angle ππ΅π΄ are equal. In particular, since angle ππ΄π·
is a right angle, we have the measure of angle ππ΄π΅ is equal to 90 minus the
measure of angle π΅π΄π·.

Now, all we need to use is the fact
that the measure of angle π·π΅π is the sum of the measure of angle π΄π΅π· and the
measure of angle ππ΄π΅. We would substitute these into our
expression and then simplify. And we would be able to find the
following result. The measure of angle π΅π΄π· is
one-half the measure of the arc from π΄ to π΅. To do this, letβs clear some space
and go back to the following equation.

We can find an expression for the
measure of angle πΆπ΅π΄ from our diagram. Angle πΆπ΅π΄ is subtended by the
major arc from π΄ to πΆ. And the measure of an inscribed
angle is one-half the measure of the arc itβs subtended by. So this is one-half the measure of
the arc from π΄ to πΆ. We can substitute our expression
for the measure of angle π΅π΄π·, giving us the following equation, which we can
rearrange for the measure of angle π΅π·π΄, which gives us the following. The measure of angle π΅π·π΄ is
one-half the measure of the major arc from π΄ to πΆ minus the measure of the arc
from π΄ to π΅.

An easy way to remember this is the
measure of the angle is one-half the difference of the measures of the two arcs
intercepted by the sides of the angle. And of course we take the positive
value for this difference.

Before we finish, thereβs one more
property we can show. Weβve already shown the measure of
the angle between two tangents of a circle which meet at a point is 180 degrees
minus the measure of the minor arc between the two points of tangency. We can relate this to our other
results by considering the measure of the other arc; letβs call this π¦. These two arcs make up a full
circle, so the sum of their measures is 360 degrees. Subtracting π₯ from both sides of
the equation gives us π¦ is 360 degrees minus π₯. And we want to use this to consider
one-half the difference between these two arcs. Thatβs one-half π¦ minus π₯.

Weβll substitute this expression
for π¦ into one-half the difference. This gives us one-half 360 minus π₯
minus π₯, which if we simplify is 180 degrees minus π₯, which by using our first
result is the measure of angle π΄πΆπ΅. In other words, we can also think
of the measure of the angle between two tangents which meet outside of a circle as
one-half the difference between the two arcs between the points of tangency.

Letβs go over the key points of
this video. First, we saw if two chords
intersect at a point in the circle, then the measure of the angle between the two
chords is half the sum of the measures of the two arcs opposite the angle. Next, we saw if two secants, two
tangents, or a secant and a tangent intersect at a point outside of a circle, then
the measure of the angle between them is half the positive difference between the
measures of both arcs intercepted by the sides of the angle. Finally, we saw the measure of the
angle between two tangents which intersect outside of a circle is 180 degrees minus
the measure of the minor arc between the two points of tangency.