\(=2\left(2+1\right)+2\left(3+1\right)+3\left(4+1\right)+...+97\left(98+1\right)+98\left(99+1\right)\\ =1\cdot2+1+2\cdot3+2+3\cdot4+3+...+97\cdot98+97+98\cdot99+98\\ =\left(1\cdot2+2\cdot3+3\cdot4+...+98\cdot99\right)+\left(1+2+3+...+98\right)\\ =323400+4851\\ =328351\)
1.3 + 2.4 + 3.5 +....+ 98.100
= 1(2+1) + 2.(3+1) + 3.(4+1) +...+ 98(99+1)
= 1.2 + 1 + 2.3 + 2 + 3.4 + 3+....+ 98.99 +98
= (1.2 + 2.3 + 3.4+....98.99) + (1+2+3+....+98)
=323400 + 4851
=328251
1.3+2.4+3.5+...+98.100
= 1.(2+1)+2(3+1)+3(4+1)+...+98(99+1)
=1.2+1+2.3+2+3.4+3+...98.99+98
=(1.2+2.3+3.4+...98.99)+(1+2+3+..+98)
=323400+48251 ( dãy số cách đều)
=32851.
