S3=S8 怎么证明出3a1+3d=8a1+28d

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S3=S8怎么证明出3a1+3d=8a1+28dS3=S8怎么证明出3a1+3d=8a1+28dS3=S8怎么证明出3a1+3d=8a1+28dS3=a1+a2+a3=a1+a1+d+a1+2d=3a

S3=S8 怎么证明出3a1+3d=8a1+28d
S3=S8 怎么证明出3a1+3d=8a1+28d

S3=S8 怎么证明出3a1+3d=8a1+28d
S3=a1+a2+a3=a1+a1+d+a1+2d=3a1+3d
S8=a1+a2+a3+...+a8
=a1+a1+d+a1+2d+...+a1+7d
=8a1+(d+2d+...+7d)
=8a1+28d
∴3a1+3d=8a1+28d