怎么用简便方法算990+991+992……1007+1008+1009=?
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![怎么用简便方法算990+991+992……1007+1008+1009=?](/uploads/image/z/10339960-40-0.jpg?t=%E6%80%8E%E4%B9%88%E7%94%A8%E7%AE%80%E4%BE%BF%E6%96%B9%E6%B3%95%E7%AE%97990%2B991%2B992%E2%80%A6%E2%80%A61007%2B1008%2B1009%3D%3F)
怎么用简便方法算990+991+992……1007+1008+1009=?
怎么用简便方法算990+991+992……1007+1008+1009=?
怎么用简便方法算990+991+992……1007+1008+1009=?
990+991+992……1007+1008+1009
=1+2+3+.+989+.+1007+1008+1009-(1+2+3+.989)
=(1+1009)×1009÷2-(1+989)×989÷2
=505×1009-495×989
=505×(1000+10-1)-495×(1000-10-1)
=505000+5050-505-495000+4950+495
=10000+10000-10
=19990