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Trig functions cross multiplying?

  1. Nov 29, 2012 #1
    trig functions cross multiplying??

    1. The problem statement, all variables and given/known data
    sinx/cosx - 2sinxcosx/1


    2. Relevant equations
    none??


    3. The attempt at a solution
    when I cross multiply, should it be sinx-2sinxcosx/cosx or sinx-2sinxcos^2x/cosx ??
    here's a pic:
    http://tinypic.com/r/24fgvmv/6
     
  2. jcsd
  3. Nov 29, 2012 #2

    Mark44

    Staff: Mentor

    Re: trig functions cross multiplying??

    You should forget you ever heard the term "cross multiply" since it is interfering with your understanding.

    What you have is the difference of two fractions and you need to carry out the subtraction. Both addition and subtraction work the same way: find a common denominator, and once you have the common denominator in both fractions, do the subtraction.

    There is no "cross multiply" in either operation.

    For example, if I need to simplify 3/5 - 1/4, I would do this:

    $$ \frac{3}{5} - \frac{1}{4}$$
    $$ =\frac{3}{5}\cdot\frac{4}{4} - \frac{1}{4}\cdot\frac{5}{5}$$
    $$ =\frac{12}{20} - \frac{5}{20} = \frac{7}{20}$$
     
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