An inscribed angle in a circle is an angle whose vertex is on the top of the circle (on the circumference of the circle) and whose ends are chords in a circle.
Therefore, if you draw any two chords in a circle, they will meet at the same point on the circumference - On the circle itself, we will create an angle. The angle that will be formed, will be an inscribed angle in the circle.
Key Properties:
Inscribed angles that subtend the same arc from the same side are equal to each other.
Equal inscribed angles stand opposite equal chords and equal arcs.
An inscribed angle measures half the angle of the central angle that subtends the same arc.
Angles inscribed in semicircles always measure 90°, creating a right angle.
When two inscribed angles lean on the same chord but from opposite sides, their measures add up to 180°
We are here to define for you what an inscribed angle in a circle is. Also, to give you tips to remember its definition and characteristics in the most logical way. Before talking about the inscribed angle in the circle, let's take a moment to look at its name - inscribed angle. Its name, lets us understand that it has a connection with the circumference and indeed it does.
Now, we can move on to the definition of an inscribed angle and it will stay in our minds thanks to logic. What is an inscribed angle in a circle? An inscribed angle in a circle is an angle whose vertex is on the top of the circle: on the circumference of the circle and its ends are chords in the circle. Let's see it in the figure:
We have a circle in front of us. We mention that an inscribed angle is an angle whose vertex is on the circle, that is, on the circumference. and whose ends are chords in a circle. Therefore, if you draw any two chords in a circle, they will meet at the same point on the circumference - On the circle itself, we will create an angle. The angle that will be formed, will be an inscribed angle in the circle. We will mark A at some point on the circle and two chords inside the circle that meet at point A.
Now that we know what an inscribed angle in a circle is and can easily identify it, we must know some important theorems and properties of an inscribed angle in a circle. Shall we start? Equal inscribed angles When can we determine that the inscribed angles in a circle are equal?
Inscribed angles that subtend the same arc from the same side are equal to each other.
That is, if there are any chords on which inscribed angles from the same side lean, they will be equal.
Let's see this in the figure:
Here is a circle and a chord AB. We can see that angles 1,2,3 lean on the chord AB from the same side and therefore are equal.
Example of angles leaning on the same chord but not from the same side:
We can see that angles 1 and 2 do lean on the same chord but not from the same side and therefore we cannot determine that they are equal.
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Test your knowledge
Question 1
In which of the circles is the point marked in the circle and not on the circumference?
Equal inscribed angles stand opposite equal chords and equal arcs.
That is, if we are given that there are equal inscribed angles, we can determine that the chords and the arcs they intercept are also equal. Let's see this in the figure:
Before us is a circle if we are given that: ∢1=∢2 Then we can determine that: AB=DC and also AB=DC
Facing equal arcs in a circle, we find equal inscribed angles.
That is, if we are given equal arcs in a circle, we can determine that the inscribed angles opposite them are equal. Let's see this in the figure:
Before us is a circle. If we are given that: AB=CD then ∢1=∢2
Now we will study the relationship between an inscribed angle in a circle and a central angle in a circle. Remember that a central angle in a circle is an angle whose vertex is at the center of the circle and whose ends are radii in the circle. Like here:
Do you know what the answer is?
Question 1
Where does a point need to be so that its distance from the center of the circle is the shortest?
The relationship between an inscribed angle and a central angle in a circle
In a circle, the inscribed angle will be half of the central angle that leans on the same arc.
That is: If in the circle we identify a central angle and an inscribed angle that lean on the same arc, we can say that as they lean on the same arc, the inscribed angle will be equal to half of the central angle. Let's see this in the figure:
The relationship between the diameter and the inscribed angle.
Remember, the diameter is the largest radius in a circle: a line that connects 2 points on the top of the circle and passes through the center of the circle. What is the relationship between this and the circumferential angle? Excellent that you asked. An inscribed angle that leans on a diameter equals 90° degrees. In the same way, we can say that if any inscribed angle in a circle equals 90° degrees, it leans on a diameter. Let's see this in the figure:
If the diameter AB then ∢ACB=90° in the same way, if ∢ACB=90° then AB is the diameter.
Check your understanding
Question 1
Calculate the length of the arc marked in red given that the circumference is 12.
Two inscribed angles in a circle that subtend the same chord from different sides
Do you remember we talked about the fact that inscribed angles leaning on the same chord on one side are equal? Now, we are talking about inscribed angles leaning on the same chord but on its two different sides. The two angles together add up to 180° degrees. Let's see this in the figure:
In front of us, there is a circle and a chord AB The angles ∢ACD and ∢ADB are inscribed angles that lean on the same chord on its two different sides and therefore their sum will be 180°.
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Inscribed Angle in a Circle (Examples and Exercises with Solutions)
Exercise #1
A point whose distance from the center of the circle is _______ than the radius, is outside the circle.
Step-by-Step Solution
Let's remember that the circle is actually the inner part of the circumference, meaning the enclosed area within the frame of the circumference.
Therefore, a point whose distance is greater than the center of the circle will necessarily be outside the circle.
Answer
greater
Exercise #2
In which of the circles is the point marked in the circle and not on the circumference?
Video Solution
Step-by-Step Solution
Let's remember that the circular line draws the shape of the circle, and the inner part is called a disk.
Therefore, in diagram B, the point is located in the inner part, meaning inside the disk.
Answer
Exercise #3
Where does a point need to be so that its distance from the center of the circle is the shortest?
Step-by-Step Solution
Let's remember that the circle is actually the inner part of the circumference, meaning the enclosed area within the frame of the circumference.
Therefore, a point whose distance is less than the radius from the center of the circle will necessarily be inside the circle.
Answer
Inside
Exercise #4
A circle has the following equation: x2−8ax+y2+10ay=−5a2
Point O is its center and is in the second quadrant (a=0)
Use the completing the square method to find the center of the circle and its radius in terms of a.
Step-by-Step Solution
Let's recall that the equation of a circle with its center at O(xo,yo) and its radius R is:
(x−xo)2+(y−yo)2=R2Now, let's now have a look at the equation for the given circle:
x2−8ax+y2+10ay=−5a2 We will try rearrange this equation to match the circle equation, or in other words we will ensure that on the left side is the sum of two squared binomial expressions, one for x and one for y.
We will do this using the "completing the square" method:
Let's recall the short formula for squaring a binomial:
(c±d)2=c2±2cd+d2We'll deal separatelywith the part of the equation related to x in the equation (underlined):
x2−8ax+y2+10ay=−5a2
We'll isolate these two terms from the equation and deal with them separately.
We'll present these terms in a form similar to the form of the first two terms in the shortcut formula (we'll choose the subtraction form of the binomial squared formula since the term in the first power we are dealing with is8ax, which has a negative sign):
x2−8ax↔c2−2cd+d2↓x2−2↓⋅x⋅4a↔c2−2↓cd+d2Notice that compared to the short formula (which is on the right side of the blue arrow in the previous calculation), we are actually making the comparison:
{x↔c4a↔d Therefore, if we want to get a squared binomial form from these two terms (underlined in the calculation), we will need to add the term(4</span><spanclass="katex">a)2, but we don't want to change the value of the expression, and therefore we will also subtract this term from the expression.
That is, we will add and subtract the term (or expression) we need to "complete" to the binomial squared form,
In the following calculation, the "trick" is highlighted (two lines under the term we added and subtracted from the expression),
Next, we'll put the expression in the squared binomial form the appropriate expression (highlighted with colors) and in the last stage we'll simplify the expression:
x2−2⋅x⋅4ax2−2⋅x⋅4a+(4a)2−(4a)2x2−2⋅x⋅4a+(4a)2−16a2↓(x−4a)2−16a2Let's summarize the steps we've taken so far for the expression with x.
We'll do this within the given equation:
x2−8ax+y2+10ay=−5a2x2−2⋅x⋅4a+(4a)2−(4a)2+y2+10ay=−5a2↓(x−4a)2−16a2+y2+10ay=−5a2We'll continue and do the same thing for the expressions with y in the resulting equation:
(Now we'll choose the addition form of the squared binomial formula since the term in the first power we are dealing with 10ay has a positive sign)
(x−4a)2−16a2+y2+10ay=−5a2↓(x−4a)2−16a2+y2+2⋅y⋅5a=−5a2(x−4a)2−16a2+y2+2⋅y⋅5a+(5a)2−(5a)2=−5a2↓(x−4a)2−16a2+y2+2⋅y⋅5a+(5a)2−25a2=−5a2↓(x−4a)2−16a2+(y+5a)2−25a2=−5a2(x−4a)2+(y+5a)2=36a2In the last step, we move the free numbers to the second side and combine like terms.
Now that the given circle equation is in the form of the general circle equation mentioned earlier, we can easily extract both the center of the given circle and its radius:
In the last step, we made sure to get the exact form of the general circle equation—that is, where only subtraction is performed within the squared expressions (emphasized with an arrow)
Therefore, we can conclude that the center of the circle is at:O(xo,yo)↔O(4a,−5a) and extract the radius of the circle by solving a simple equation:
R2=36a2/→R=±6a
Remember that the radius of the circle, by its definition is the distance between any point on the diameter and the center of the circle. Since it is positive, we must disqualify one of the options we got for the radius.
To do this, we will use the remaining information we haven't used yet—which is that the center of the given circle O is in the second quadrant.
That is:
O(x_o,y_o)\leftrightarrow x_o<0,\hspace{4pt}y_o>0 (Or in words: the x-value of the circle's center is negative and the y-value of the circle's center is positive)