Question

已知公差不为0的等差数列{an}的首项a1＝3，若a2，a3，a6成等比数列，则{an}的前n项和

已知公差不为0的等差数列{an}的首项a1＝3，若a2，a3，a6成等比数列，则{an}的前n项和第四题

Answer 1

图

Answer 2

an = a1+(n-1)d
若a2，a3，a6成等比数列
a2.a6=(a3)^2
(a1+d)(a1+5d)= (a1+2d)^2
(3+d)(3+5d)= (3+2d)^2
5d^2+18d=4d^2+12d
d^2+6d=0
d=-6
an = 3-6(n-1)=9-6n
Sn = a1+a2+...+an
= n(6-3n)
= 6n -3n^2
= 3-3(n-1)^2
max Sn = S1 =3
ans :A

追问

锐角三角形ABC的三个内角A,B,C的对边分别为a,b,c,a＝1B＝2A,则b/cosA＝,b的取值范围是

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