Question

第四题怎样做求具体解题

Answer 1

a3^2=a2×a5,
(a1+2d)^2=(a1+d)(a1+4d),
4a1d=5a1d,
a1=0,(因d不为0),
ak=s20,
a1+(k-1)d=(a1+a20)×20÷2,
(k-1)d=19d×10=190d,
k-1=190,
k=191

追答

选C

Answer 2

由已知：a3²=a2•a5
(a1 + 2d)²=(a1 + d)(a1 + 4d)
a1² + 4a1d + 4d²=a1² + 5a1d + 4d²
a1d=0
∵d≠0
∴a1=0
∵ak=S20
∴(k-1)d=20•19•d÷2
(k-1)d=190d
∵d≠0
∴k=191