等差数列sm=30,s2m=100,s3m=?

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等差数列sm=30,s2m=100,s3m=?等差数列sm=30,s2m=100,s3m=?等差数列sm=30,s2m=100,s3m=?若{an}为等差数列则,sn,s2n-sn,s3n-s2n,s

等差数列sm=30,s2m=100,s3m=?
等差数列sm=30,s2m=100,s3m=?

等差数列sm=30,s2m=100,s3m=?
若{an}为等差数列则,sn,s2n-sn,s3n-s2n,s4n-s3n.为等差数列这个是很重要的,你记下,不过,老师应该说过吧
所以这道题：
a1+a2+a3+...+am=30 ①
a1+a2+a3+...+am+...+a2m=100 ②
②-①得
(am+1)+(am+2)+...+(a2m)=70 ③
所以①喝③的公差为70-30=40
所以 (a2m+1)+(a2m+2)+...+a3m=110 ④
则s3m=a1+a2+...+am+(am+1)+...+(a2m)+(a2m+1)+...+a3m
=①+③+④
=30+70+110
=210

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