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: Did i do it correct

  1. Jan 4, 2004 #1
    Ok So i finished my questions and was wondering if someone could check my work to see if i did it right.
    Question: If CosA= 1/3, with 0<A<pie/2, and SinB=1/4, with pie/2<B<pie, Calculate sin(A+B)

    My Solution:

    Sin(A+B)= SinACosB + CosASinB
    =SinACosB + (1/3)(1/4)
    SinACosB + 1/12

    Ok Now i drew Triangle A and labeled the bottom 1 and hypotaneous 3. Then used the theorum to get the last side is square root 8.
    I made Triangle B and labelled hypotaneous 4, adjacent side being 1 and used theorum to get the opposite side to be square root of 15.

    =(square root8/3)(square root15/4) +1/12
    =(square root120/12) + 1/12
    to get my final answer of squareroot 120 +1 /12.

    Is that correct?
  2. jcsd
  3. Jan 4, 2004 #2


    User Avatar
    Science Advisor

    Ouch! Your trigonometry is excellent- your notation is horrible!

    You are exactly right (and for the right reasons!) that
    sin(A+B)= =(&radic;(8)/3)(&radic;(15)/4) +1/12

    but then you seem to have lost control of your parentheses.

    sin(A+ B)= &radic(120)/12+ 1/12
    = (&radic(120)+ 1)/12

    which is not the same as &radic(120)+ 1/12 which is how I would have interpreted "squareroot 120 +1 /12".

    It's not your use of "squareroot" instead of &radic; I am complaining about- it's your use of parentheses.

    (squareroot(120)+ 1)/12 would be a correct answer.
  4. Jan 4, 2004 #3
    shouldn't cosB be negative for [tex] \frac{\pi}{2}<B< \pi [/tex]
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook