D=71+72+73+74+...+72010
=(71+72+73)+(74+75+76)+...(72008+72009+72010)
=(7.1+7.7+7.72)+(74.1+74.7+74.72)+...+(72008.1+72008.7+72008.72)
=7.(1+7+72)+74.(1+7+72)+...+72008.(1+7+72)
=7.57+74.57+72008.57
=57.(7+74+...+72008)
=>D chia hết cho 57
D = 7+72+73+.....+72000
D = (7+72+73)+(74+75+76)+....+(71998+71999+72000)
D = 7(1+7+72)+74(1+7+72)+....+71998(1+7+72)
D =7.57 + 74.57 +......+ 71998.57
D = 57.(7+74+....+71998) chia hết cho 57 (đpcm)