忘忧草萱
幼苗
共回答了9个问题采纳率:100% 举报
1-2+3-4+5-6+.+2007-2008+2009
=(1-2)+(3-4)+.+(2007-2008)+2009
=-1+(-1)+(-1)+.+(-1)+2009
=(-1)*2008/2+2009
=1005
1/3+1/6+1/10+1/15+1/21+1/28
= 2*(1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56)
= 2*[1/(2*3) + 1/(3*4) + 1/(4*5) + 1/(5*6) + 1/(6*7) + 1/(7*8)]
= 2*(1/2 - 1/3 + 1/3 -1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8)
= 2*(1/2 - 1/8)
= 3/4
(4/7*10/9*4/11)/(2/11+2/7+5/9) = 0.22566995768688
1年前
9