Question

等差数列中S5=1 S10=5 求S20

Answer 1

等差数列的求和公式Sn=nA1+[n(n-1)d]/2

由题意可以列出方程式组

1=5*A1+[5*(5-1)*d]/2

5=10*A1+[10*(10-1)*d]/2

解得A1=-1/25,d=3/25

S20=22