PreCalculus - Trigonometry: The Law of Cosines (11 of 15) The Law of Cosines Proved

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In this video I will prove The Law of Cosines.
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3:26 cos180 is -1 and cos ( -A) is cosA so clever sir thank you

vdrux
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Thank you, Sir. But can this law be proven with a triangle that has no angles more than 90°?

احسانملکی-فث
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Why does sin of 180-A equal A, I am stuck?

racehallabrin