đề câu b sai rồi ,như thế này mới đúng :
b .12+22+32+...+982
giải:
ta có: a=1.2+2.3+3.4+...+98.99
b=12+22+32+...+988
.=1.1+2.2+3.3+...+98.98
=(1.2-1.1)+(2.3-2.2)+(3.4-3.3)+...+(98.99-98.98)
=1(2-1)+2(3-2)+3(4-3)+...+98(99-98)
=1.1+2.1+3.1+...+98.1
=1+2+3+...+98
=[98.(98+1)] / 2
= 98 .99 / 2
= 4851
vậy :a-b = 4851
a = 1 . 2 + 2 . 3 + 3. 4 + .... + 98 . 99
3a = 1 . 2 . 3 + 2 . 3 . ( 4 - 1 ) + 3 . 4 . ( 5 - 2 ) + .... + 98 . 99 . ( 100 - 97 )
3a = 1 . 2 . 3 + 2 . 3 . 4 - 1 . 2 . 3 + ... 98 . 99 . 100 - 97 . 98 . 99
3a = 98 . 99 . 100 = 970200
a = 323400
a = 1 . 2 + 2 . 3 + 3. 4 + .... + 98 . 99
3a = 1 . 2 . 3 + 2 . 3 . ( 4 - 1 ) + 3 . 4 . ( 5 - 2 ) + .... + 98 . 99 . ( 100 - 97 )
3a = 1 . 2 . 3 + 2 . 3 . 4 - 1 . 2 . 3 + ... 98 . 99 . 100 - 97 . 98 . 99
3a = 98 . 99 . 100 = 970200
a = 323400