General term formula of arithmetic sequence The sum of the preceding terms of the arithmetic sequence {an} is SN. It is known that A10 = 20, S20 = 410 is the general term formula of the arithmetic sequence {an} If = 155, find n If Sn = 155, find n

General term formula of arithmetic sequence The sum of the preceding terms of the arithmetic sequence {an} is SN. It is known that A10 = 20, S20 = 410 is the general term formula of the arithmetic sequence {an} If = 155, find n If Sn = 155, find n

{an} is the arithmetic sequence, A10 = 20, S20 = 410
So A10 = a1 + (10-1) d = 20
S20=20a1+[20*(20-1)*d]/2 ②
① A 1 and D can be obtained by a set of equations
The general term formula an = a1 + (n-1) d substituted into arithmetic sequence‘
The general formula an