I, too, would refuse to post an answer there. The alternative tricks leading to a solution (plane rotations or use of complex roots of unity) have been used/posted/thoroughly dissected on our site on several occasions.
It is an "obvious" FAQ.
Noobs are welcome to try their hand at finding a good explanation. Six-digit-veterans should spot it as an obvious dupe pretty much on sight (or expect to be called out as rep-farmers). Don't know where the line is in-between?