Trigonometry: The Law of Cosines

[funkiet0es]

you are a wonderful human being

[MRMONKEYPUNCH]

the only reason im passing math is this guy thumbs up!

[kary1982v6]

THANK YOU all of the other vids use an angle.. I needed 3 sides and no angle, you showed both ways..

[beanieman117]

at 5:00, you need to put a negative in front of (2ac), so it should look like -2ac

[perlazulmanzanita]

How did he solve it in the calculator?? I CANT!!! what happen with the cos???

[TheSpartanfan]

@perlazulmanzanita You do the inverse of the COS (cos ^ -1) and enter "-0.677". You'll get 128 (rounded to three sig figs.)

[jacSMACs]

thank you, creepy math man.



[yoyoimut]

you should choose simpler numbers.

[plastig002]

thanks so much. id love to have you as a math teacher!

[bomrdude]

This is where babies come from.

[xEmoLov3rx]

Mrs. Pefley's geometry class is the bomb.

[gounc93]

what about when checking for a second triangle? That is where i screw up...

[imadaddysgurl15]

THANK YOU SO MUCH :)

[JotaLegendOfficial]

Thanks Doc!!! I got a big test tomorrow, and this helped a lot!!

[plastig002]

@JotaLegendOfficial haha same

[TheLockerzAddict]

the brackets 

[meghnacool434]

very helpful

[Fifa0verCod]

i was watching this for a minute, went to get a snack, came back an hour and a half later...

[AngelicPocketwatch]

thank you for the video it helped a lot

[plastig002]

@sTyreEuREka click mode

[sTyreEuREka]

how can i set my calculator in the degree mode????

[LEGENDPURGE]

@sTyreEuREka

Go to the back of your calculator, there should be a very small button. Use a pin/sharp pencil to press it, this will reset it into Degree mode. (Well, it does with mine anyhow).

[dyeballa04]

@sTyreEuREka take apart your entire calculator.....then reassemble it with a banana on your head...it should be in degree mode then

[josephmendoza16]

340Sin(151.5)/727 you get.2231554037... make sure you are in degree mode

[xxawesomenestxx]

@josephmendoza16

[raery14]

how did he get the 0.2232?.please can some one tell me

[alfredoa86]

law of sines. solve for one side and multiply your anwser by the denominator of the unsolved side... sinC / 340 = sin 151.5 / 727( solve for this side first) = 6.5634. now multiply the 6.5634 by  the denominator of the unsolved side ( 340) .. ( 340 x 6.5634) = .2232

[AngelicPocketwatch]

@raery14 You have to cross multiply. To do this you mulitply 340 X sin(151.5) Then divide by 727 and the answer should be .2232

[pauleeeee09]

The 10 people who disliked this must be so foolish not to understand this excellent tutotial! =)

[n4gauge]

thx you and dont mind these idiots commenting bad or complaining they dont get it

theyr just super idiot evrything hir i understand others they want is spoonfeeding

[moose1976o]

inverse of COS .1667 is = to 80 degrees

WHEN STUDYING MATH REMEMBER THAT THERE ARE MANY MISTAKES

IN THE BOOKS AND EXAMPLES. BUT END RESULT IS THE SAME NO MATTER WHAT METHOD YOU USE. ITS ABOUT LEARNING HOW TO MANIPULATE NUMBERS.

[jojosh234]

what is the difference of S.A.S AND A.S.S?? it's kinda the same, anyways can you give a tip if you'll gonna use law of sine or law of cosine in solving the sides and the angles

[bevon17]

eff it im moving on.... I think your #'s are wrong because I am doing what you're doing but my answers are not matching up @ 5:00

[bevon17]

Why can't I get

COS A correct???

I can't get -0,6177

I've tried 1000 times

[moose1976o]

BECAUSE THE ANSWER IS 80 DEGREES NOT 128

[bevon17]

WTF YOU SKIPPED A STEP!!

please stop skipping the steps!!

at 5:00 you skipped something WTFFFFFFFFFFFFFFFF

God math makes me want to commit suicide

[bevon17]

OK SOOOO

I guess when you look for an angle you don't divide it by SIN(727), but 727 by its self...

not sure why, 

[bevon17]

OK SO

COS is for Side/Angle/ Side??

and Sin is for Angle/ angle/ side??

[halflife1994]

you may rest in peace

[AutopsyReport22]

My trig. teacher is mediocre. He is conceited and spends most of class time talking about his wife and kids. I'm glad I have this man helping me. At 5:00 it would help if you go through the steps of how you transformed the law of cosine equation. Some may not know how you came to that equation as it does not originally start like that. thnx.

[bobbycornacchia]

It's really amazing...

Youtube is completely revolutionizing education, because instead of requiring some terrible teacher to repeatedly lecture me on this...I have this teacher clearly explaining all I need to know.

What's really exciting is when Youtube realizes this and gets their act together I could completely homeschool myself. Also, the best teachers will probably rise to the top and will become the most influential, just like Athenian Academia!

[deanbassist01]

rrrrrrrgg WHY DOES NOBODY KNOW HOW TO DO IT IF YOU HAVE SIDE A AND ANGLES B AND C!!!!!!! I CANT FIND ANYBODY WHO EXPLAINS HOW TO DO THAT!!!!

[danaemmashow]

@deanbassist01 you use the law of sines

[quefnot]

@deanbassist01 find the third angle by subtracting angles b and c from 180, and then use the law of sines

[crazzynez]

you are way better than my ugly old boring trig teacher who isnt any good

[frenZy6200]

LEGEND WITH A D

[tydude789]

totally having to re-learn all my trig functions and stuff with oblique triangles before my semester final! Thank you so much! I didn't know you had to take the answer to the inverse sine. My teacher doesn't include that in her notes. You're the man!

[santoyobarber]

you are a good man..thanks

[benthekoolest]

Dude i have a huge test in math tomorrow and i have a dumb preganant teacher so this saves my life!!! Thank You!

[icortes09]

thank you so much.....this helped me a lot.

[netk91]

thank you sir

[MrTheGoodin]

I really appreciate u posting this on YouTube. You seriously saved my life.

[Stantheman282]

old man are you on crack cocaine or other narcotics because you suck at math!

[timlip12]

Awesome vid, very helpful! I hate trying to learn from just a textbook, especially since they often skip little steps. Granted they often are prerequisites to the section, but it's easy to forget stuff when we spend so much time on different sections that don't have much to do with another and jump back and forth.

[lukeofdarth]

Thx to this dude i got an A on my quiz today!

[rbd7]

thank u so much, I don't know where teachers like u go? u r great :)

[alicexoxfull]

how i wish you are my teacher

[bilalmukhlis]

how do u find the angle using the law of cosines if you are given 3 sides of the triangle and no angle??????

[marcusFOREVER]

you can't get the exact number only proportions...

[rundmc525]

woww your this is so helpful. my teacher stupid

[blacklightning98]

i don't understand how he got 727 m. I have been trying to work it out on my calculator but I keep getting 196 m.

[DSNCB919]

enter it piece by piece and write it down.. most likley you are entering it into the calculator wrong which is a common mistake

[b98154]

@blacklightning98 Side B cannot be 196 m. It has to be the greatest side in the triangle since angle B is the greatest angle in the triangle. Therefore, you should check your format of the problem on the calculator.

[xxmtbxtremexx]

this is so confusing... thanks for the help

[ACDYZG]

ah this saved me i didnt get it at all

[YaarZePirate]

thnx ur a great teacher ;)

[fanofwrestling123]

This is much better than reading my notes. Have my final exam Next Thursday.

[scottau190]

@ericleb01 - There are several variations of the Law of Cosines formula all derived from a^2 = b^2 + c^2 - 2bcCosA. You can find them in any precalc book. It is like the Pythagorean theorem can be written as a^2 + b^2 = c^2 OR we could say a^2 = c^2 - b^2. Basically, you start off with a^2 = .... Subtract b^2 and c^2 from each side then divide by -2bc and you will be left with Cos A = ...

[ericleb01]

I don't understand where he got the CosA = b^2 + c^2 - a^2 / 2bc. Somebody want to help explaining?

[wheelienick]

this guy isn't really that good.

[myfriday9694]

tnx

[HopamHR]

it does... like it does matter if you use P or p for Power(beucase p is presure).... you use A,B and C for 2 things.... it's stupid. Someone asks: "HEY, HOW BIG IS A??!!" and there are 2 answers in your case ;)

[HopamHR]

Americans.... have to be special :( Why don't you use alpha, beta and gamma for angles, like the rest of the world.

[0633767jerishowrulez]

it does really matter what you call the angles A or alpha

[braydenbeautiful]

what difference does it make?

None.

Use theta, alpha, beta -- it makes no difference what so ever. They're just unknowns.

[burnxthiscitydown]

thank you so much :]

[androknroll]

I've seen a bunch of these videos and this guy's the best one.

[el4ndrez]

His eyebrows are the best. HAHAHAHA

Inverse cosine on TI-84 is 2nd (the blue button) then Cos.

[neoblood3d]

Thanks again!

[rgrybra]

Thank you!

[precalcskipper]

i absolutely love all your videos.

[KEveleigh13]

i cant get the answer 528 714

[schase10353]

because you've got to solve for b right away, if you use his way then you need to put it all under the radical first. It's the only way I get it...

[changalator]

me either, I got 71101.42246

[lukeofdarth]

@changalator maybe u got ur calc on radian mode

[chudoki]

@changalator I got the same thing. did u figure out what was wrong?

[BboyBRad07]

2 x 410 x 340 = 278,800

151.5 cos = -0.878817113

278,800 x -0.878817113 = -245,014.211

-----------------------

410 squared + 340 squared = 283,700

(2 minus' equal plus) so

283700 + 245014.211 = 528,714.211

he took off the 0.211

=) hope it helped! peace!

[juscuzimawesome]

@KEveleigh13 you have to set your mode to degrees instead of radians

[babienikkay]

you press 2nd COS

and it will show up as Cos -1

[inyuyasha160]

where is the inverse cosine button???

How do I use it???

I have a TI-84 plus.

Please help me!

[prosno666]

do you have a shift button or a 2nd function button? if so you hit that button then cos and you have inverse cosine. hope it helps.

[charizardsspirit]

thanks dude

[backtolife15]

=D thanks to you im getting an A

[kudakadere]

some people dont realise that the cosine rule is the full version of pythagors' theorem......that second part of the formula is always left out because in a right angled triangle , the cosine is zero

hope someone finds this helpful :D

[khussett]

the problem i have are the world problems, i would prefer if my professor gave us a drawn triangle indicating the sides and angles already.

[laloconqueso]

i wish i was still in geometry

trigonometry is confusing!

[nessensu]

lol i solved it but i ust have done it weird cuz i got:

429372.5998

[MusicGuru12]

Thanks (:

[CollyStrings]

This is a SAS (side angle side) triangle. Not as hard as the ambiguous ASS (angle side side) triangle.

[jacobbis4lovers]

Lol @ ASS

[LongWayShout]

really thanky you

are one of the best teacher's of the world

x)

[Trajedya84]

wow dis iz awesome d00d!!! :))

[SELGX]

goood vid

[vsuperville]

great video

[vsuperville]

great video

[muchi090]

Thank you white man u helped me allot

[loler4eva]

@muchi090 lol "you white man" ?_?

[meghnacool434]

@muchi090 racist -.-

[rosemariemay]

@muchi090 Racist black girl, learn to spell.

[TEDtomo]

thanks that was the most helpful video of the law of cosines i have seen

[Pistonian008]

yeah it helped me to pass the entrance exam at Colegio De San Juan De Letran

Arriba!!!

thanks

[nycsharkman]

What about law of cosine word problems?

[nycsharkman]

What about some law of sines and law of cosines word problems?

[goodoneron]

thanks a lot this was a great help. i now know what im doing

[paindeer]

Would prefer more 'as to WHY X, Y or Z is ?' in understanding sine, cosine, tan.

BUT - this helped. I'm curious as to why it's 2AC cos B

I guess I am looking for how or why the law of sine, or law of cosine exists.