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anonymous2419 · Nov 18, 2010, 06:02 PM

please verify this equation:

cos(x+y+z) = cos(x)cos(y)cos(z) - sin(x)sin(y)cos(z) - sin(x)cos(y)sin(z) - sin(x)sin(y)sin(z)

Unknown008 · Nov 19, 2010, 04:22 AM

You could use the compound angle formula:

$cos(A + B) = cosAcosB - sinAsinB$

Let A = x+y and B = z

$cos(x + y+ z) = cos(x + y)cos(z) - sin(x+ y)sin(z)$

Use the same identity again:

$sin(A+B) = cosAsinB + sinAcosB$

$(cos(x)cos(y) - sin(x)sin(y))cos(z) - (cos(x)sin(y) + sin(x)cos(y))sin(z)$

I'll let you expand and see by yourself :)