由题意有:
(1)S4=10
(2)(a1+2d)^2=(a1+d)(a1+6d)
联立得:
d=0或d=-(3/2)a1
则:当d=0时
a1=5/2
当d=-(3/2)a1时
a1=10,d=-15
则,通项公式为an=5/2或an=10-15(n-1)
由题意有:
(1)S4=10
(2)(a1+2d)^2=(a1+d)(a1+6d)
联立得:
d=0或d=-(3/2)a1
则:当d=0时
a1=5/2
当d=-(3/2)a1时
a1=10,d=-15
则,通项公式为an=5/2或an=10-15(n-1)