有关数列请写出过程,谢谢
a3=a1+2d----------------------①
a5=a1+4d=a1+2d+2d=6-----------②
a5∶a3=(a1+4d)∶(a1+2d)=6∶(a1+2d)
a1²+6a1d+8d²-6a1-12d=0=====>
(a1+3d)²=d²+6a1+12d=(d+6)²-36+6a1======>
(a1+3d)²-(d+6)²=6(a1-6)=====>
(a1+4d+6)(a1+2d-6)=6(a1-6)======>
①|a1+4d+6=6,a1+2d-6=a1-6=====>a1=...全部
a3=a1+2d----------------------①
a5=a1+4d=a1+2d+2d=6-----------②
a5∶a3=(a1+4d)∶(a1+2d)=6∶(a1+2d)
a1²+6a1d+8d²-6a1-12d=0=====>
(a1+3d)²=d²+6a1+12d=(d+6)²-36+6a1======>
(a1+3d)²-(d+6)²=6(a1-6)=====>
(a1+4d+6)(a1+2d-6)=6(a1-6)======>
①|a1+4d+6=6,a1+2d-6=a1-6=====>a1=-24,d=6
②|a1+4d+6=a1-6,a1+2d-6=6=====>a1=18,d=-3
∴a3=-24+2×6=-12,a5=-24+24=0(不合题意,舍去)
∴取第②组值:a3=18-2×3=12,a5=18-4×3=6(符合题意)
∴(a5)²=a3×a(n1)=====>
a(n1)=3
。
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