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

cos(3π/4+x) + sin (3π/4 -x) = 0

= 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.

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