简便计算（要过程，能看懂）
第1个回答 2012-04-04
（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
=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
