1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Proving angle sum trig identies w/ vector and scalar products

  1. Oct 4, 2012 #1
    1. The problem statement, all variables and given/known data

    I need to prove both of these (in exercise 11)

    [PLAIN]http://postimage.org/image/x7shxv11f/ [Broken][/PLAIN]

    2. Relevant equations

    The dot product

    3. The attempt at a solution

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 4, 2012 #2
    What do you mean where did you go wrong? Isn't that what you were asked to prove? You really need to write down some intermediate steps though. It's impossible to follow your "proof" as it is now. Now you just wrote down the starting point and the end result.
  4. Oct 4, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Your first mistake is in the very first line: What is ##\vec{A}\vec{B}\cos(\theta+\phi)## supposed to mean? You can't just stick two vectors next to each other like that. ##\vec{A}\cdot\vec{B}## makes sense because it's saying your dotting the two vectors together; similarly, ##\vec{A}\times\vec{B}## makes sense because it's saying you're taking the cross product. ##\vec{A}\vec{B}## doesn't mean anything.

    Your second mistake is that you're using the very identity you're trying to prove. So far, you're using the fact that ##\vec{A}\cdot\vec{B} = |\vec{A}||\vec{B}|\cos(\theta+\phi)##. You need to find a second way to express the dot product.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook