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Homework Help: How do I manipulate this?

  1. Dec 5, 2014 #1
    1. The problem statement, all variables and given/known data
    I just want to manipulate this equation

    2. Relevant equations
    8 - {[(4(rcos(x))^2 +4(rsin(x)^2)^2] / 2}

    I should have:


    3. The attempt at a solution

    I used FOIL and the Pythagorean identity to get:

    8- {[16r^4 + (32r^4sin(x)^2cos(x)^2)] / 2}

    which is wrong
  2. jcsd
  3. Dec 5, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Use the basic trig identity involving ##\sin^2x## and ##\cos^2 x## before expanding.
  4. Dec 5, 2014 #3


    Staff: Mentor

    That's not an equation. The tipoff that it isn't an equation is that no = sign appears anywhere in this expression.
  5. Dec 5, 2014 #4
    Thanks, I got the answer. Much easier than what I did the 1st time.

    Attached Files:

  6. Dec 5, 2014 #5
    It's part of an equation.

    Attached Files:

  7. Dec 5, 2014 #6


    Staff: Mentor

    Not relevant.

    What you said was "I just want to manipulate this equation." but what you showed is not an equation. That was my point. It's not a major thing, but it is important to get the terminology right so as to be able to communicate effectively.
  8. Dec 5, 2014 #7


    User Avatar
    Gold Member

    Also, I believe you typed your equation wrong.
    you typed:
    8 - {[(4(rcos(x))^2 +4(rsin(x)^2)^2] / 2}
    I think you meant to type
    8 - {[(4(rcos(x))^2 +4(rsin(x)^2)]^2 / 2}
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