等比等差的問題 20點

1. 7*14 = 98
7*15 = 105
...
7*142 = 994
7*143 = 1,001

所有可被 7 整的三位數之和
= 7*15 + 7*16 + ... + 7*142
= 7* (15+16+...+142)
= 7 * (15 + 142 ) * (142 - 15 +1)/2 (等差數列和為頭加尾乘項數除2)
= 70,336

thank you so much!