Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple algerba

  1. Aug 14, 2007 #1
    ok this is in an example in a first-year physics textbook, yet some how i spent an hour trying to solve it (identities, tricks, etc) and could not find a solution.

    here is what the textbook states:
    (1) v2f*cos[tex]\phi[/tex] = 3.5*10^5 - v1f*cos(37)
    (2) v2f*cos[tex]\phi[/tex] = v1f*sin(37)

    "now we sqaure these two equations and add them"

    = 1.23*10^11 - 7*10^5*v1f*cos(37) + v1f^2*cos(37)^2 + v1f^2*sin(37)^2

    (then it simplifies further)

    I know that 1.23*10^11 is (3.5*10^5)^2 and that 7*10^5 is 2*3.5*10^5

    any help in explaining how this equation is derived would be a great help (ESPECIALLY how 7*10^5 seems to pop out of no where)
    Last edited: Aug 14, 2007
  2. jcsd
  3. Aug 14, 2007 #2
    Did you try squaring and adding the terms?
  4. Aug 14, 2007 #3
    More importantly, where did that cos(37) come from?
  5. Aug 14, 2007 #4
    sorry the cos 35 is supposed to be cos 37. my mistake.

    Yes i did try adding them and squaring them. your point? why the **** would i take the time to ****ing type it down clearly for your convenience.
  6. Aug 14, 2007 #5
    You obviously missed something doing that. Remeber that -2ab when expanding (a-b)^2? :grumpy:
  7. Aug 14, 2007 #6
    lol, you know what... i didn't forget that i should get -2ab... i made an even dumber mistake... i did (a^2 - b^2)

    wow... thanks for pointing it out though, holy crap.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook