[tex]\bf \cfrac{cos(x+y)}{cos(x-y)}=\cfrac{cot(x-y)}{cot(x+y)} \\\\[-0.35em] ~\dotfill\\\\ \stackrel{\textit{expanding the left-hand-side}}{\cfrac{cos(x)cos(y)-sin(x)sin(y)}{cos(x)cos(y)+sin(x)sin(y)}} \\\\[-0.35em] ~\dotfill[/tex]
[tex]\bf \stackrel{\textit{expanding the right-hand-side}}{\cfrac{~~ \frac{cos(x)cos(y)+sin(x)sin(y)}{sin(x)cos(y)-cos(x)sin(y)}~~}{\frac{cos(x)cos(y)-sin(x)sin(y)}{sin(x)cos(y)+cos(x)sin(y)}}} \\\\\\ \cfrac{cos(x)cos(y)+sin(x)sin(y)}{sin(x)cos(y)-cos(x)sin(y)}\cdot \cfrac{sin(x)cos(y)+cos(x)sin(y)}{cos(x)cos(y)-sin(x)sin(y)}[/tex]
