等差数列{An}满足7a5=-5a9,且a1=-17求数列{An}的前n项和Sn的最小值请各位快点给出回应，

Question

等差数列{An}满足7a5=-5a9,且a1=-17
求数列{An}的前n项和Sn的最小值请各位快点给出回应，

Answer 1

7a5=-5a97*（a1+4d)=-5(a1+8d)7a1+28d=-5a1-40d12a1=-68d6a1=-34d则 6*（-17）=-34dd=3an=-17+3(n-1)an=-20+3nSn=-20n+3n(n+1)/2=1/2(3n^2-37n)=-1/6 3n(37-3n) =-1/6 （√【3n(37-3n) 】)^2>=-1/6 (37/2)^2此时3n=3...