Multiplication using dot product.

AryRezvani

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.

gneill

Check the value you've calculated for A.B

AryRezvani

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.

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.

AryRezvani

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!

gneill

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}##?

AryRezvani

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.

gneill

Quote by AryRezvani

Hahaha thanks, took me a while to catch that.. sadly.

Glad I could help

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AryRezvani	Multiplication using dot product. Tweet 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.

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

gneill 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.