560-557+554-551+……+500-497

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560-557+554-551+……+500-497560-557+554-551+……+500-497560-557+554-551+……+500-497560.557.554.551.……500.

560-557+554-551+……+500-497
560-557+554-551+……+500-497

560-557+554-551+……+500-497
560.557.554.551.……500.497 为公差为-3的等差数列
497=560-3(n-1)
n=22
n'=n/2=11
Sn=11*3=33

=3+3+3+......+3
一共有(560-500)/6=10个3=30

3*60=180

560-557+554-551+……+500-497=(560-557)+(554-551)+……+(500-497)。((560-500)/6+1)*3=33