For a right-angled triangle sin(x)=O/H [where O is the side opposite the angle x and H is the hypotenuse] while cot(x)=A/O [where A is the side adjecent to x and H is again the hypotenuse.
Therefore sin(x)cot(x)=O/H*A/O=OA/HO=A/H. Cos(x)=A/H. So sin(x)cot(x)=cos(x).