- #1

- 10

- 0

whats the angle between Avec and Bvec in rads.

i know the formula

A(vec)*B(vec)= abs(A)*abs(B)*cos(theta)

A(vec)*B(vec)=-10

so why wont cos^-1(-10/(21)(10)) work?

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- Thread starter philo51
- Start date

- #1

- 10

- 0

whats the angle between Avec and Bvec in rads.

i know the formula

A(vec)*B(vec)= abs(A)*abs(B)*cos(theta)

A(vec)*B(vec)=-10

so why wont cos^-1(-10/(21)(10)) work?

- #2

quasar987

Science Advisor

Homework Helper

Gold Member

- 4,783

- 18

absA = [itex]\sqrt{21}[/itex], not 21

- #3

- 10

- 0

that still isnt right?

- #4

HallsofIvy

Science Advisor

Homework Helper

- 41,833

- 963

You were told that the length of vector A is [tex]\sqrt{21}[/tex], not 21.

Did it then occur to you that the length of vector B should be [tex]\sqrt{10}[/tex], not 10?

Try [tex]\theta= cos^{-1} \frac{-10}{\sqrt{210}}[/tex].

- #5

- 10

- 0

no i realized that i just multiplied weird.. sorry thanks!

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