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!

Trigonometry Obscurity!

  1. Sep 5, 2004 #1
    Hey everyone,
    I was looking over some old pre-calculus exams and I found this rather obscure looking question.. It's about trigonometry.

    You're given a triangle ABC, and the legs are a (BC),b (AC), c (AB).
    You're given the lenghts of a=5, b-8, and the angle C between them is 140.

    The question is, what's the length of the leg "c" and what are the other 2 degrees? :yuck:

    Is that even possible?? :bugeye:
  2. jcsd
  3. Sep 5, 2004 #2
    Are you familiar with the sine and cosine rules?
  4. Sep 5, 2004 #3
    Of course I am!! :grumpy:
    But they can only be applied to a triangle with a right angle, and I tried to divide this triangle to 2 with right angles but it just didnt seem to work out.. Any ideas?
  5. Sep 5, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    devious is referring to the law of sines and the law of cosines...

    Given any triangle with sides A, B, and C with angles [itex]\alpha, \beta, \gamma[/itex] (with A opposite [itex]\alpha[/itex], etc):

    The law of sines:
    \frac{\sin \alpha}{A} = \frac{\sin \beta}{B} = \frac{\sin \gamma}{C}

    The law of cosines:
    C^2 = A^2 + B^2 - 2AB\cos \gamma

    (and, of course, similar formulae for the other choices of angle)
    Last edited: Sep 5, 2004
  6. Sep 5, 2004 #5


    User Avatar
    Science Advisor
    Homework Helper

    Notice that since cos(90) = 0, the law of cosine turns into pythagoras' (sp?) theorem with right triangles. That's the rule I assume you were talking about.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook