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设 x 1 , x 2 , x 3 , x 4 , x 5 , x 6 , x 7 是自然数,且 x 1 $amp; x 2 $amp; x 3 $amp; x 4 $amp; x 5 $amp; x 6 $amp; x 7 , x 1 + x 2 = x 3 , x 2 + x 3 = x 4 , x 3 + x 4 = x 5 , x 4 + x 5 = x 6 , x 5 + x 6 = x 7 ,又 x 1 + x 2 + x 3 + x 4 + x 5 + x 6 + x 7 =2010,那么 x 1 + x 2 + x 3 的值最大是 。