解决三齿轮重合问题，假设三个齿轮在某一瞬间重合，问经各过多少圈后再次重合。齿轮数分别为a，b，c-To solve the coincidence of three gears, assuming three gears in a moment of coincidence, and asked how many laps in all the overlap again. Gears, respectively for a, b, c