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January 27, 2015

January 27, 2015

Posted by **Kiely** on Thursday, November 29, 2007 at 11:05pm.

= cos(3π/4)cosx + sin(3π/4)sinx + sin(3π/4)cosx - cos(3π/4)sinx

= -1/sqrt2cosx + 1/sqrt2sinx + 1/sqrt2cosx - (-1/sqrt2sinx)

I canceled out -1/sqrt2cosx and 1/sqrt2cosx

Now I have

1/sqrt sinx + 1/sqrt2 sinx

And that doesn't equal 0. So where did I go wrong?

Also cos(x+y)cosy + sin(x+y)siny = cosx

I ended up with

(cosxcosy) + sinxsinycosy + (sinxcosy) + cosxsin^2y

I don't know what to do next.

- Math help again -
**Michael**, Thursday, November 29, 2007 at 11:26pmTry your sum and difference identities again. I don't believe you've expanded them correctly.

cos(x + y) = cosx * cosy - sinx * siny

cos(x - y) = cosx * cosy + sinx * siny

sin(x + y) = sinx * cosy + cosx * siny

sin(x - y) = sinx * cosy - cosx * siny

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