Trig functions cross multiplying?

[musiclover55]

trig functions cross multiplying??

Homework Statement

sinx/cosx - 2sinxcosx/1

Homework Equations

none??

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

 

[Mark44]

Homework Statement

sinx/cosx - 2sinxcosx/1

Homework Equations

none??

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

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:

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