rr小K
幼苗
共回答了17个问题采纳率:88.2% 举报
(1)原式=5(1/2+1/6+1/12+1/20+1/30)
=5(1/1*2+1/2*3+1/3*4+1/4*5+1/5*6)
=5(1/1-1/2+1/2-1/3+.-1/6)
=5*5/6=25/6
2题是等比数列 Sn=a1(1-q^n)/(1-q)
=1/2(1-1/2^6)/(1-1/2)
=1-1/64
=63/64
3题 同样的道理5/6=1/2+1/3, 7/12=1/3+1/4.
代人即得答案 11/18
1年前
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