Question

1×5÷1+5×9÷1+9×13÷1+...+2005×2009÷1等于多少?

Answer 1

1×5÷1+5×9÷1+9×13÷1+...+2005×2009÷1

=n*(n+4)+````

= [5*9*13-1*5*9]/12+```

=[2005×2009-1*5*9]/12

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