牛势冲天
幼苗
共回答了22个问题采纳率:95.5% 举报
1.
原式=(3-1)+(7-5)+(11-9)+……+(195-193)+(199-197)
=2+2+2……+2+2(共50个,算法,3,7,11都是差4个,所以199-3=196,196/4=49,再+1,共50个.)=2*50=100
2.拆项:
原=1-(1/2+1/3)+(1/3+1/4)-(1/4+1/5)+(1/5+1/6)-(1/6+1/7)
=1-(1/2)-(1/3)+(1/3)+(1/4)-(1/4)-(1/5)+(1/5)+(1/6)-(1/6)-1/7
=1-1/2-1/7
=1-9/14
=5/14
1年前
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