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Argument(Angle from dot product)

caseclosed
in Analysis and Calculus

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cos(theta)= (A.B)/(lAllBl) OR

cos(theta)=(lA.Bl)/(lAllBl) ---->my teacher insist this one is right and other one

is wrong

Clearly, your teacher is wrong - as cosine can be negative, and hir answer can't be, that answer is wrong.

The one question about yours is whether, when the angle is greater than 90, the dot product is positive or negative. If you get positive, then your answer is wrong too. Otherwise, your answer is right.

=Uncool=

I suppose the question boils down to asking what [math]\theta [/math] actually is supposed to be. So: What is it? Are you sure it´s the same as what your teacher defined it to be?

 

EDIT: @uncool: It´s only clear if you know what [math]\theta [/math], A and B are. And that is absolutely not obvious to me.

  • Author

well, I just did the questions in the book and my professor's method doesn't match the answer in the book. He says the book is wrong......

the book is Thomas's Calculus 11th edition.

 

also I see nowhere online where it says absolute value of dot product.

  • 2 weeks later...
  • Author

ok, I have claried this, apprarently it was for angle between 2 planes... grr those conventional ways

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