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+28d](/uploads/image/z/4927653-45-3.jpg?t=S3%3DS8+%E6%80%8E%E4%B9%88%E8%AF%81%E6%98%8E%E5%87%BA3a1%2B3d%3D8a1%2B28d)
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