(1) 1. arithmetic progression: the general formula an=a 1+(n- 1)d, the first term a 1, the tolerance d, a AK = AK+(n-k)d AK, the nth term is If A, A and B constitute arithmetic progression, then A. Then Sn = Na 1+N (n-1) D/2 = DN 2 (that is, the quadratic power of N) /2+(a 1-d/2)n has the following summation methods:/kloc. Geometric series: the general formula an = a 1 * Q (n- 1) (that is, the n- 1 power of q) a 1 is the first term, and an is the nth term An = A668. Am = a 1 * q (m- 1), then an/am = q (n-m) (1), and an = am * q (n-m) (2). If a, g and b constitute an equal proportion term, then. Sn = a1+a2+a3 ... an Sn = a1+a1* q+a1* q 2+... a1* q (n However, it is difficult to derive some questions with the following formula. This time may be directly derived from the basic formula, so I hope this formula can be understood. )Sn = a 1( 1-q N)/( 1-q)=(a 1-an * q)。 Note: Q is not equal to1; Sn=na 1 Note: q= 1 There are generally five ways to sum: 1, complete induction (that is, mathematical induction), 2- multiplication, 3- dislocation subtraction, 4- reverse summation and 5- split term elimination.