3(9X + 5Y)= 27X + 15Y = (13X + 8Y) + (14X + 7Y) = (13X + 8Y) + 7(2X + Y) 7能整除(13X + 8Y)、7(2X + Y) 即证得7能整除 3(9X + 5Y),因此 7能整除 9X + 5Y.
