奥数2011-2001+1991-1981+1971-1961+···+51-41+31-21=?
2011-2001+1991-1981+1971-1961+···+51-41+31-21=(2011-2001)+(1991-1981)+(1971-1961)+......
2011-2001+1991-1981+1971-1961+···+51-41+31-21= (2011 - 2001) + (1991 - 1981) + (1971 - 1961) + ... + (51 - 41) + (31 - 21) = 10 + 10 + 10 + ... + 10 + 10 = 10 *100 = 1000
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