Law of cosines to determine grade | Trig identities and examples | Trigonometry | Khan Academy

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[@new8218]

a teacher might take a whole week of explaining this and I still don't understand, but this guy did it in under 7 minutes and made it look so simple

[@leilanivallejo6271]

Hi, I binge watched all the sine, cosine, and Law of (Co)sine videos. And I got an A *100% on my test. My grade went from a B to an A in geometry. THANK YOU Khan Academy!!!

[@nicholasmartinez5104]

I love this guy with all my heart, savior of math and science!!

[@JohnnyGraves_]

A much easier way of doing it would be to do the square of both adjacent sides (to which ever angle you wanted to find) added, then take away the opposite side squared. Then make that the numerator over 2 X one of the adjacent angles X the other adjacent angle. So it would look like this: 50^2 + 60^2 - 20^2 --------------------------- 2 X 50 X 60 Which would then give you the 19/20, and then you just do Cos^-1 (19/20) to give you the answer of 18.19487.... or just 18.2.

[@fanmajnekrofta27]

Thanks mate you saved me from a headache

[@jamalkhalil4750]

You legit just saved a life

[@jandy608]

I think the traditional way is easier though

[@quasarhunter4423]

Mans really made me pass geometry

[@mathewdede1004]

I love you Khan Academy man🥺

[@hyplexlgz4133]

Why not just use the formula for cosine rule angles : CosC = a^2 + b^2 - c^2/ 2 x a x b

[@osooso9069]

Thanx 👍🏿

[@wesmesser4006]

y does he suddenly take the inverse cosine at the end?

[@TitusWang375]

wes messer I know this is a really late reply, but the reason he takes the the inverse cosine of both sides is because you can't leave your answer in terms of cosine. In order to rid of the cosine, you have to take the inverse cosine of both sides.

[@wesmesser4006]

Ah like algebra thanks that clears that up nicely

[@TitusWang375]

No problem this is one of the only things I happen to understand in my intro to Calculus AB but Sal is doing a great job teaching regardless

[@davisatdavis1]

How do I find the sides of a triangle when I have the cosine but not the sides?

[@IBeToxic]

i gotchu To use this formula to find the sides of a triangle, you need: The length of at least two sides of the triangle and the cosine of the included angle to find the third side. The length of one side and the cosine of one of the non-adjacent angles, but you'll also need additional information (such as another angle or another side) to use the Law of Cosines effectively. If you only know the cosine of an angle without any side lengths, you cannot directly find the specific lengths of the sides of the triangle without more information. Additional details such as another side length or another angle might be necessary to solve for the sides. Could you provide more details about what exactly you know regarding the triangle? For example, do you know the length of any sides, or do you have the cosine of an angle and the length of the side opposite that angle? Any additional information would be crucial for solving for the sides.

[@jerry6102]

ok, I am a little confused now. If you forget the title of the video and now you wouldn't know what we are talking about. So now how do you know that you have to take the law of Cosines to solve for the angles? Why not law of Sin or Tangent.

[@JLsLEGOWorkshop]

Law of tangent is for right triangles and law of sine is only usable when you know an angle and the length of opposite of the angle

[@SirDinzhi]

im confused since when did 6000 become negative?

[@psymonbaker]

Because of -2ab :)

[@NuEnque]

Why did you (from out of no where) subtract 6100 from C2? c2 = a2 + b2 - 2(AB) cos0

[@firethylacine1976]

NuEnque He did it to get rid of 6100 on the right side

[@dylanhogan6660]

why can't you be my trig teacher?

[@Sebastian-xy3xk]

So many views, so little comments

[@TitusWang375]

Helloz World True lol

[@answertheask4191]

What app or a special computer he fuckins use

[@NuEnque]

He's using photoshop and a Wacom tablet (to write/draw). You can tell when the color pallet appears on screen from time to time.

[@wasayrehman5712]

Or he's using Microsoft paint with a Wacom tablet