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: How do I evaluate this properly? Trig

  1. Jan 9, 2005 #1
    I have the question evaluate the following : cos(-pi/12)

    I rewrote this to be -15 degrees = 345 degrees.

    I think Im supposed to use the sum and difference formula or special triangle, so I wrote

    cos(30degrees-45degrees)= cos 30 cos 45 - sin 30 sin 45

    my answer turned out to be 0.5124 but it says the correct answer is 0.9659 what did I do wrong?
  2. jcsd
  3. Jan 9, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    U applied the 'cosine' formula incorrectly.It should read
    cos(30degrees-45degrees)= cos 30 cos 45 +sin 30 sin 45

    I think it gives you the right answer.
  4. Jan 9, 2005 #3
    Ok I changed the sign but now my answer is 1.219 it is still wrong
  5. Jan 9, 2005 #4
    You added wrong, aisha
  6. Jan 9, 2005 #5
    yes ur right thanks i didnt do it separately I just input the whole thing into the calculator thanks :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook