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.根据下面一组等式 S 1 =1 S 2 =2+3=5 S 3 =4+5+6=15 S 4 =7+8+9+10=34 S 5 =11+12+13+14+15=65 S 6 =16+17+18+19+20+21=111 S 7 =22+23+24+25+26+27+28=175 … … … … … … … … 可得 .
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