Law of Sines

The law of sines is all about proportions, so make sure to keep yours in order.

FunctionsProve and apply trigonometric identities
LanguageEnglish Language
Right Triangles and TrigonometryLaw of Sines and Law of Cosines

Transcript

00:31

Dr. Dorito tells security that two of his lengths are 3.2 and 4, and that his angle

00:38

A is 60 degrees. Given Angle-Side-Side, to figure out the rest

00:44

of the angles and side lengths, they'll use the law of sines.

00:50

Sine A over a equals sine B over b.

00:55

We can plug in A as 60 degrees, and side length A as 4.

01:00

The only other side we have is 3.2, so that'll be b.

01:05

We have sine of 60 degrees over 4 equals sine of the angle at point B over 3.2, the side

01:12

length of b. To solve for angle B, we can multiply both

01:16

sides by 3.2. We want B, not sine B. So if we take the inverse sine of each side, we

01:31

see that B is around 43.9 degrees. Wonderful. Now we have two out of three angles

01:38

and two out of three sides. Knowing that all the angles in a triangle

01:43

add up to 180° allows us to find the last angle.

01:48

Substitute in the angles we know. A as 60 degrees and B as 43.9 degrees.

01:55

To solve for C, we can subtract 60 and 43.9 from both sides, to get C equals 180 degrees

02:04

minus 60 degrees plus 43.9 degrees... ...to get that C equals 76.1 degrees.

02:13

One side to go. We can use either the ratio between sine 60 degrees and 4 or sine 43.9

02:20

degrees and 3.2. Let's use the ratio between sine 60 degrees

02:25

and 4. Substitute what we know. We're looking for

02:29

the last side, c. To solve for side length c, we can rearrange

02:34

the equation by cross-multiplying. Sine sixty degrees times c equals 4 times

02:40

sine 76.1 degrees. To isolate c, we can divide both sides by sine sixty degrees...to get

02:53

that c equals around 4.5. Double-checkin' time. Do we know all of Dr.

03:00

Dorito's angles and all the sides? Yup, sure do. All three side lengths and all

03:05

three angles. Now they just have to make sure he's not smuggling

03:09

any zesty taco flavoring across the border...