等差数列sm=70,s2m=300,求s3m

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等差数列sm=70,s2m=300,求s3m
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等差数列sm=70,s2m=300,求s3m
等差数列sm=70,s2m=300,求s3m

等差数列sm=70,s2m=300,求s3m
sm,s2m-sm,s3m-s2m成等差数列
sm=70,s2m-sm=300-70=230
s3m-s2m=2*(s2m-sm)-70=460-70=390
s3m=390+300=690

530

根据等差数列特点:sm+s3m=2*s2m
s3m=2*s2m-sm
=2*300-70
=530

因为2(S2m+Sm)=Sm+S3m
所以2*(300+70)=70+S3m
解得S3m=570

530