# multiplying trig functions

• Feb 17th 2008, 02:57 PM
heroic
multiplying trig functions
could you help me on the following,

rcosxcosy * rsinxcosy

-rsinxsiny * sinxsiny
• Feb 17th 2008, 05:19 PM
TwistedOne151
$r\cos{x}\cos{y}\cdot{r}\sin{x}\cos{y}=r^2\sin{x}\c os{x}\cos^2{y}$
$-r\sin{x}\sin{y}\cdot\sin{x}\sin{y}=-r\sin^2{x}\sin^2{y}$

-Kevin C.
• Feb 18th 2008, 03:08 AM
heroic
cheers mate.

now could you possibly help me simplify the following plz,

cosx[(r^2sinxcos^2y) - (r^2sinxcosxsin^2y)]

(-rsinx)[(rsin^2xcos^2y) - (-rsin^2xsin^2y)]

I'm sorry i'm askin for help, i kno this must be easy, but i can't do it. lol
• Feb 18th 2008, 03:31 AM
Peritus
try using the following identities:

$
\begin{gathered}
\cos ^2 x - \sin ^2 x = \cos 2x \hfill \\
2\sin x\cos x = \sin 2x \hfill \\
\sin ^2 x + \cos ^2 x = 1 \hfill \\
\end{gathered}
$

you might also take a look at this:

Table of Trigonometric Identities