Proof of angle addition formula for cosine | Trigonometry | Khan Academy

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[mathew_pang]

this is the great video!! I love the way you teach! thank you so much =)

[Rocky-mecw]

If i didn't know the proof and someone asked me to do i could never do this.
I do mean "NEVER".

[karljo]

But this proof only applies to situations where both angles are acute angles and their sum is an acute angle, otherwise the triangle doesn't hold. How can you extend this formula to all kinds of angles?

[OtekpoCollins-msiq]

I have been having problem with deriving the proof, but your videos just taught me an easy pattern of deriving it.i wish we can have tutors like this in our school

[MrAkura]

But how do you prove it if x or y are equal or greater than 90 degrees? And what about the negative degrees?
Do you have to prove all over again every specific case?

[kaseymitsuri]

It makes so much sense when Sal says it, and how he goes at everything step by step. Although just thinking it by ourselves is very veeery difficult. I need more practice xD
But it's soo cool to watch this videos and understand! Thank you, Khan Academy!!

[simplelistbuildingformula]

Wow, this is a great refresher to trigonometry that I learned years ago...thanks for the mathematical proofs!

[roseb]

anyone have any ideas how i can remember this

[seandafny]

yea but who the hell would just randomly draw two triangles on top of each other in just the right way for this to come out neatly. I'm beginning to realize that alot of genius just comes from boredom.