等差数列an中,Sm=t,S2m=s,求Sm+n应该是Sm=t,Sn=s,求Sm+n

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等差数列an中,Sm=t,S2m=s,求Sm+n应该是Sm=t,Sn=s,求Sm+n等差数列an中,Sm=t,S2m=s,求Sm+n应该是Sm=t,Sn=s,求Sm+n等差数列an中,Sm=t,S2m

等差数列an中,Sm=t,S2m=s,求Sm+n应该是Sm=t,Sn=s,求Sm+n
等差数列an中,Sm=t,S2m=s,求Sm+n
应该是Sm=t,Sn=s,求Sm+n

等差数列an中,Sm=t,S2m=s,求Sm+n应该是Sm=t,Sn=s,求Sm+n
Sm=a1+(m-1)d=t
Sn=a1+(n-1)d=s
t-s=(m-n)d
Sm+n=a1+(m+n-1)d=a1+(m-1)d+nd=t+nd=t+n(t-s)/(m-n)

是求S3m吗？
既然是等差数列，那Sm，S2m-Sm，S3m-S2m也是等差数列
2(S2m-Sm)=Sm十S3m-S2m
2s-2t=t十S3m-2s
S3m=4s-3t

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