{an}是等差数列,公差为1 a1+a2+\\\\\+a99=99,a3+a6+a9+\\\\\+a99=

{an}是等差数列,公差为1 a1+a2+\\\\\+a99=99,a3+a6+a9+\\\\\+a99=
{an}是等差数列,公差为1 a1+a2+\\\\\+a99=99,a3+a6+a9+\\\\\+a99=

{an}是等差数列,公差为1 a1+a2+\\\\\+a99=99,a3+a6+a9+\\\\\+a99=
等差数列的通项公式是
an=a1+(n-1)d a1+a2+...+a99
=a1+a1+d+a1+2d+...+a1+98d
=99a1+d+2d+...+98d
=99a1+98(98+1)/2d
=99a1+4851=99 a1
=(99-4851)/99=-48
a3+a6+a9+...+a99
=33a1+2d+5d+8d+...+98d
=-33*48+1650 =66

a1+a2+\\\\\+a99=(a1+a1+98)*99/2=99
2a1+98=2
a1=-48
a2=-47
a3=-46
a3.a6.a9....这个数列可以看成首项为a3,差为3的等差数列:
a3+a6+a9+\\\\\+a99=(-2*46+32*3)*33/2=66