题目内容
(文)Sn=1-2+3-4+5-6+…+(-1)n+1•n,则S100+S200+S301等于( )
A.1 | B.-1 | C.51 | D.52 |
∵Sn=1-2+3-4+5-6+…+(-1)n+1•n,
∴S100=1-2+3-4+5-6+…+(-100)=(1-2)+(3-4)+…+(99-100)=-1×50=-50,
S200=1-2+3-4+5-6+…+(-200)=(1-2)+(3-4)+…+(199-200)=-1×100=-100,
S301=1-2+3-4+5-6+…+301=1+(3-2)+(5-4)+…+(301-300)=1+150=151,
∴S100+S200+S301=-50-100+151=1,
故选:A.
∴S100=1-2+3-4+5-6+…+(-100)=(1-2)+(3-4)+…+(99-100)=-1×50=-50,
S200=1-2+3-4+5-6+…+(-200)=(1-2)+(3-4)+…+(199-200)=-1×100=-100,
S301=1-2+3-4+5-6+…+301=1+(3-2)+(5-4)+…+(301-300)=1+150=151,
∴S100+S200+S301=-50-100+151=1,
故选:A.
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