6+11+16---+5011+5+9+13+---+1989+1993

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6+11+16---+5011+5+9+13+---+1989+1993
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6+11+16---+5011+5+9+13+---+1989+1993
6+11+16---+501
1+5+9+13+---+1989+1993

6+11+16---+5011+5+9+13+---+1989+1993
等差数列求和:(首项+尾项)*项数/2
这两题需要算的主要是项数,项数=(尾项-首项)/公差+1
6+11+16+……501=(6+501)*[(501-6)/5+1]/2=25350
1+5+9+13……1993=(1+1993)*[(1993-1)/4+1]/2=497503

(前项+后项)*项数/2
(1+1993)*398/2=396806

=497503

(首项+末项)*项数/2
(1+1993)*499/2=497503