The Cosine Law

This lesson shows how to use the Cosine Law to find both sides and angles in triangles. This lesson was created for the MCR3U Functions course in the province of Ontario, Canada.

Category:

Education

Tags:

• MCR3U
• Functions

[StupidKidsFiles]

how do u get an angle when you know ALL of the sides but no angles??

[AlRichards314]

@StupidKidsFiles When you are given three sides, but no angles you use the formula solved for Cos of an angle. See the first page on the video, the three formulas in red on that page. There is an example on the second page of the lesson showing how to find an angle given 3 sides.

[theonlyreega]

what if you have 2 angles and 1 side given?

[AlRichards314]

@theonlyreega If you have 2 angles given and a side, then you really have all 3 angles (sum to 180) and a side. This isn't the senario where you can use the Cosine Law since you need 2 sides to find the third one using Cos Law. However, when you have 3 angles and a side you can use the Sine Law to find the other 2 sides. Actually, you could use the Sine law to find a second side and then once you have 2 sides and 3 angles you could use either the Cos or Sine Laws to find the remaining side.

[zaqqy2010]

not sure how u got that 55 as the answer for cosC

[AlRichards314]

@zaqqy2010 In the second example cosC = 0.5676, so C = cos^-1(0.5676) = 55 degrees. The cos^-1 function on a scientific calculator gives the angle when you know the cosine decimal value. Make sure your calculator is in degree mode and not radians or gradients.

[00Mj000]

u know what, McDonald's here i come

[shill61]

Finally, finally i get it i took you 10 minutes to show me what that dumb bitch has been talking about for days

[JokotaJonas10]

Thanks so much!!! :) reviewing for my final and your videos are huge help!!

[308728823]

U DA BEST MAN XD

[monguiller]

who are those 2 dislikers XOO i hate you two!!! this tutorial is the best!!!!!!

[profuckenguy]

man i think my teachers been teaching me the wrong shit for a week

[msyangmath]

Great clear video. Forgive me if I missed this somewhere in the video but did you go over the disclaimer: If given SSS then find largest angle first, and if given SAS, find the third side, then the smaller angle first?

If not, then problems occur where the math is correct but the solution simply is not. Example for SSS, a=5, b=7, c=10. If you solve for A first, it leads to a wrong angle. For SAS, A = 50 degrees, b=12,c=5. If you solve for B first, then it would lead to a wrong angle.

[ArdinTheCreaToR]

you are are the fucking man, saved my ass with this shit, OFWGKTA