Question

有谁 1＋4＋7＋10＋13＋.＋331＋334的和是奇数还是偶数

Answer 1

等差数列通项:an=a1+(n-1)d

等差数列前n项和:Sn=na1+[n(n-1)/2]d

等比数列通项:an=a1q^(n-1)

等比数列前项和:Sn=a1(q^n-1)/(q-1)

易知 d = 3

由an = a1 + (n-1)d

得n = 112

所以 Sn = na1 + [n(n-1)/2]d

= 112 * 1 + [112*（112 - 1）/2]*3

= 18760