sin(x+y) - sin(x-y) - 1 = cos(2x)
sin(90) - sin(x-y) - 1 = cos(2x)
1 - sin(x-(90-x)) - 1 = cos(2x)
-sin(2x-90) = cos(2x)
-1*(sin(2x)cos(90) - cos(2x)sin(90)) = cos(2x)
-1*(sin(2x)*0 - cos(2x)*1) = cos(2x)
-1*(0 - cos(2x)) = cos(2x)
-1*(-cos(2x)) = cos(2x)
cos(2x) = cos(2x)
This confirms the identity is true.
Notice that throughout this proof, I only changed the left hand side.
On the 5th line, I used the identity sin(A-B) = sin(A)cos(B)-cos(A)sin(B).
