`a)` Đặt `A`

`A=1xx3 +3xx5 +5xx7 +....+99xx101`

`6A=1xx3xx(5+1) + 3xx5xx(7-1) +5xx7xx(9-3) +...+ 99xx101xx(103-97)`

`6A=3+ 1xx3xx5 +3xx5xx7-1xx3xx5 + 5xx7xx9 -3xx5xx7 +....+99xx101xx103 - 97xx99xx101`

`6A=3+99xx101xx103=1019703`

`=>A=1019703/6`

`b)` Đặt `A`

`A = 1xx2 + 2xx3 + 3xx4 + 4xx5 + ...+ 99xx100`

`A3 = 1xx2xx3 + 2xx3xx3+ 3xx4xx3 + 4xx5xx3 + ... + 99xx100xx3`

`A3 = 1xx2xx3 + 2xx3xx(4-1) + 3xx4xx(5-2) + 4xx5xx(6-3) + ... + 99xx100xx(101-98)`

`A3 = 1xx2xx3 + 2xx3xx4 - 1xx2xx3 + 3xx4xx5 - 2xx3xx4+ 4xx5xx6 - 3xx4xx5 + ... + 99xx100xx101 - 98xx99xx100`

`A3 = 99xx100xx101`

`A= (99xx100xx101)/3`

`A = 999900/3`

`=>A=333300`

Đáp án:

Giải thích các bước giải:

 câu a 

đặt a =1x3+3x5+5x7+7x9+...+99x101

6A= (1x3+3x5+5x7+7x9+...+99x101)

6A=1X3X6+3X5X6 =5X6X7=7X9X6+...+99X101X6 

6A= 1X3X6 +3X5X6 +5X6X7 +7X9X6+...+99X101X6 

6A = 1X3X5+1X3+3X5X7-3X5+5X7X9-3X5X7 +7X9X11-5X7+9+...99X101X97

6A= 3+99X101X103 

=> A= 3 +99 X101X103 PHẦN 6

CẦU B 

 ĐẶT A 

SORRY CÂU B