等差数列{An},的前n项和为Sn,已知Am-1+Am+1-(Am)*2=0,S2m-1=38,则m=?

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等差数列{An},的前n项和为Sn,已知Am-1+Am+1-(Am)*2=0,S2m-1=38,则m=?等差数列{An},的前n项和为Sn,已知Am-1+Am+1-(Am)*2=0,S2m-1=38,

等差数列{An},的前n项和为Sn,已知Am-1+Am+1-(Am)*2=0,S2m-1=38,则m=?
等差数列{An},的前n项和为Sn,已知Am-1+Am+1-(Am)*2=0,S2m-1=38,则m=?

等差数列{An},的前n项和为Sn,已知Am-1+Am+1-(Am)*2=0,S2m-1=38,则m=?
a(m-1)+a(m+1)-am ²=0
am ²=a(m-1)+a(m+1)
am ²=am -d+am+d
am ²-2am=0
am (am-2)=0
am=0或am=2
S(2m-1)=(2m-1)a1+(2m-1)(2m-2)d/2
=(2m-1)[a1+(m-1)d]
=(2m-1)am=38
am=0时,等式恒不成立.
am=2时,2(2m-1)=38
2m-1=19
2m=20
m=10
m=10

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