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Homework Help: Equation help with trig functions

  1. Oct 23, 2012 #1
    I am currently working a physics problem and I have run into some math that I don't understand.

    y = 4.0m + 4.0m(sin theta) = 4.0m(1+sin(theta))

    In the problem I am trying to find a specific height at a certain angle (pendulum problem). I have found some help online that walks me through each step of the problem. I am just trying to understand how the above equation works, specifically the last part how 4 + 4(sin (theta)) is somehow equivalent to 4(1+sin(theta)). Is this some application of a formula or simple algebra that I am not understanding?

    Maybe if you need more info you could check out the problem if you want too. The problem is located here: http://iweb.tntech.edu/murdock/books/v1chap6.pdf (Scroll down to page 143, worked examples # 6.2)
  2. jcsd
  3. Oct 23, 2012 #2


    Staff: Mentor

    This is nothing more than the distributive property.

    a*b + a*c = a * (b + c)

  4. Oct 23, 2012 #3
    Okay so that's good to know. I am still a little confused. Why does the 4.0m turn into 1? For instance if I have (4+4)(x) I get 4x + 4x which equals 8x right? So maybe this is not the distributive property but if I have (4 +4)(sin(theta)) would that not be 4sin(theta) + 4sin(theta) which = 8sin(theta).. guess I still don't understand.
  5. Oct 23, 2012 #4


    Staff: Mentor

    Here's a cleaned up version of what you write in post #1.
    When you're doing algebra, you should get rid of the units. They don't add anything useful, and can cause confusion.

    4 + 4sin(theta) = 4*1 + 4* sin(theta) = 4 * (1 + sin(theta))

    Let's make it real simple, and get rid of the trig stuff.

    4 + 4x = 4*1 + 4*x = 4(1 + x)
    Get it?
  6. Oct 23, 2012 #5
    Awesome, thank you that's exactly what I needed to see.
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