Posted by **Anonymous** on Tuesday, February 21, 2012 at 3:31pm.

Simplify:

sin(x-y)cosy+cos(x-y)siny

= (sinxcosy-cosxsiny)cosy+(cosxcosy+sinxsiny)siny

= ???

What do I do next???

Please Help and Thank You

- Trigonometry Help -
**Reiny**, Tuesday, February 21, 2012 at 5:08pm
expand it again

= sinx cos^2 y - cosxsinycosy + cosxcosysiny + sinxsin^2 y

= sinx(cos^2 y - sin^2 y)

= sinxcos(2y)

- Trigonometry Help -
**Anonymous**, Tuesday, February 21, 2012 at 5:19pm
when I substitue the x's and y's I get different answers.

do I need brackets on (sinxcosy-cosxsiny) and (cosxcosy+sinxsiny)?

could the next step be: sinxcosy-cosxsinycosy+cosxcosy+sinxsinysiny without the brackets? or would that be wrong?

- correction - Trigonometry Help -
**Reiny**, Tuesday, February 21, 2012 at 6:01pm
found my error

2nd last line should have been

= sinx(cos^2 y + sin^2 y)

= sinx (1)

= sinx

- Trigonometry Help -
**Anonymous**, Tuesday, February 21, 2012 at 6:12pm
thank you so much=)

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