Question

急!已知公差不为零的等差数列{An}中,a1=1,a1,a3,a7成等比数列.？

Answer 1

a1=1
a3=1+2d
a7=1+6d
a1,a3,a7成等比数列
a7/a3=a3/a1
1+6d=(1+2d)*(1+2d)
d=1/2或0
已知公差不为零
d=1/2
an=0.5n+05
答完收功!,10,a3=a1+2d=1+2d
a7=1+6d因为a1，a3，a7成等比数列
∴a3/a1=a7/a3
（1+2d）²=1+6d
得出d=1/2或d=0因为公差不为0 所以d=1/2
an=1+（n-1）*0.5=（n+1）/2,2,a3=a1+2d a7=a1+6d
∵a1，a3，a7成等比数列
∴(a1/a3)^2=a3/a7
代入得到8d=-12
d=-3/2
∴An=-3/2n+5/2,0,a1=1
a3=a1+2d
a7=a1+6d
a1,a3,a7等比,a3=a1q,a7=a1q^2, a1a7=a1^2q^2=a3^2
a1(a1+6d)=(a1+2d)^2
6a1d=4a1d+4d^2
a1=2d
an=a1+(n-1)d=a1+(n-1)a1/2=a1(n+1)/2=(n+1)/2,0,急!已知公差不为零的等差数列{An}中,a1=1,a1,a3,a7成等比数列.
求数列{An}的通项公式.跪求,帮忙解下,着急,在线等