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Master Angles in a Circle: Formulas, Rules, and Examples

CC.HSF.TF.A.1

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Converting between degrees and radians

Trigonometric ratios of angles in radians

Radian measure and arc length

CC.HSF.TF.A.2

Unit circle

CC.HSF.TF.A.3

Solving expressions using 45-45-90 special right triangles

Solving expressions using 30-60-90 special right triangles

Find the exact value of trigonometric ratios

CC.HSF.TF.B.5

Graphing transformations of trigonometric functions

Determining trigonometric functions given their graphs

CC.HSF.TF.B.6

Find the difference of squares: (a - b)(a + b) = (a^2 - b^2)

Find the difference of squares

Solving two-step linear equations using addition and subtraction: ax + b = c

Two-step Linear Equations

Two-step Linear Equations LHS

Solve Linear Equations Using Addition and Subtraction – Word Problem

Two-step Linear Equations — Word Problem

CC.HSF.TF.B.7

Evaluating inverse trigonometric functions

CC.HSF.TF.C.8

Pythagorean identities

CC.HSF.TF.C.9

Sum and difference identities

Sum and Difference Identities

Double-angle identities

Cofunction identities

CC.HSG.C.B.5

CC.HSG.CO.A.1

Line symmetry

Angles in a circle

Chord properties

Circles and circumference

Circle properties

Parallel and perpendicular line segments

Identifying Parallel Lines

Identifying Perpendicular Lines

CC.HSG.SRT.C.6

Use sine ratio to calculate angles and sides (Sin = o / h)

Use tangent ratio to calculate angles and sides (Tan = o / a)

CC.HSG.SRT.C.7

Use cosine ratio to calculate angles and sides (Cos = a / h)

CC.HSG.SRT.C.8

Combination of SohCahToa questions

Word problems relating ladder in trigonometry

Word problems relating guy wire in trigonometry

CC.HSG.SRT.D.10

Law of sines

Ambiguous case: SSA triangles

Law of cosines

CC.HSG.SRT.D.11

Applications of the sine law and cosine law

CC.HSG.SRT.D.9

Area of triangles: 1/2 a*b sin(C)

math

Intro

- Hi, welcome to this question right here. So, in the following diagram which is here, the radius of the triangle is 24 centimeters, right? And we know the angle B, D, and C. That's this angle right here, is 75 degrees. So, we're trying to figure out the angle BAC, which is right over here. So, that's the missing angle that you're trying to figure out, okay? 
Now, there's a relationship between this angle and this angle. Now, the angle 75 is called the central angle. The central angle is called the C, for example, okay? 
And the inscribed angle of that central angle is only just half of that, okay? So, it's equal to the inscribed angle but you need to put a two times right there, okay? 
So, basically the central angle is double, double of the inscribed angle. Now, how do you know that one is really an inscribed while the other is the center? Well, the central angle is always from a point and then the other point, and they form with the center line, okay? The center point, okay? So, this is your central angle right here. Now, if you start and finish with the same points, which is B and C, and you form any triangle around your circle right here, those are called your inscribed angles, okay? Or they have the same arc lengths. You can actually look at the arc lengths right here. This is the arc length of the triangles, right? 
So, as long as they share the same arc length. Doesn't have to start from here to here. It could be somewhere else. As long as the arc length, the A-R-C, is the same. Therefore, this equation right here will exist, okay? The central angle is the double of the inscribed angle. So, if that's 75 over here, then, basically it's just two times the missing angle. Which is, let me put down. . . 
75 degrees and there's your two theta that's missing, okay? So, just grab your calculator and take the 75 divided by two, you get yourself the angle, which is this one right here, the BAC will just be 37.5 degrees. And that's your final answer for this question. Thanks for watching.

In a circle, chords, angles, inscribed angles and arc length all have special relationships with each other. This lesson focuses on exploring the relationships among inscribed angles in a circle as well as those of inscribed angle and central angle with the same arc. We will make use of the relationships to solve related questions in this lesson.  

Using formulas to find angles in a circle

Central angles and proofs

Inscribed angles and proofs