## Multiplication using dot product.

1. The problem statement, all variables and given/known data

2. Relevant equations

Tan θ=A.B/|A|*|B|

3. The attempt at a solution

A=4i-9j
B=9i-6j

A.B=-18
Magnitude of A = √97
Magnitude of B = √117

Solve that out using Cos-1 (-18/Sqr97*Sqr117) and I keep getting the wrong answer.

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 Recognitions: Homework Help Check the value you've calculated for A.B

 Quote by gneill Check the value you've calculated for A.B
Ahh just caught that. Replugged in the values, and still came out wrong.

Work:

Plugged in 90/(Sqr97*Sqr117) = .8448190756

Tan-1 (.84481....) = 40.191 degrees.

Plugged that into online homework and it was wrong apparently.

Recognitions:
Homework Help

## Multiplication using dot product.

Check you definition of A.B in terms of magnitudes and angle. You used a correct method in the final line of your first post.

 Quote by gneill Check you definition of A.B in terms of magnitudes and angle. You used a correct method in the final line of your first post.
Could you specify? Are you referring to A.B=|A||B|Cosθ?

Edit: nevermind, got it!

Recognitions:
Homework Help
 Quote by AryRezvani Could you specify? Are you referring to A.B=|A||B|Cosθ?
Yup. The formula involves a cosine, so why are you employing ##tan^{-1}##?

 Quote by gneill Yup. The formula involves a cosine, so why are you employing ##tan^{-1}##?
Hahaha thanks, took me a while to catch that.. sadly.

Recognitions:
Homework Help
 Quote by AryRezvani Hahaha thanks, took me a while to catch that.. sadly.