(2)sn=(- 1/3)*bn+ 1, which is a formula. Sn+ 1 (it is n+ 1 item that cannot be typed out) =(- 1/3)*bn+ 1 (this is also)+1, which is the second formula. Subtract formula 1 from formula 2 to get sn+1-sn = bn =-1/3bn+1/3bn, so 2bn=-bn+ 1, so it is equal ratio.

(3)cn= 1/n, which can be obtained by substituting an, TN =1/2 (1-1/3+1/2-1/4+65438).

You can calculate for yourself which ones are left and which ones are lost. Tn =1+1/2-1/(n+1)-1/(n+2).

After simplification, it can be calculated. It's simple. Use analytical methods. You can ask again if you don't understand. Hope to adopt