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因為為3,6,9,12,所以,這是一個等差數列,公差為3,式子是3n,第一個數n=1,第二個數n=2,.所以第200個數是600. 前100數和為:(a1+a100)*100/2=15150 純手打,望樓主採納!祝樓主一生平安!幸福!
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