2+4+6+8+10一直加到198等于多少？怎么算的，要过程

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S =2+4+6+...+198 (1)
S =198+196+....+2 (2)

(1)+(2)
2S = (2+198)+(4+196)+...+(198+2)
=200+200+...+200
=200x99
S =100x99 =9900
2+4+6+...+198 = S =9900

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2+4+6+8+10+。。。+198
=2（告卖1+2+3+4+。。。+99）
=2×4950
=9900
2+4+6+8+10+。。戚友厅高隐。+198
=99×（2+198）÷2
=9900

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