1+1/(1+2)+1/(1+2+3)+1/(1+2+3+4)+……+1/(1+2+3+4……+10)等于几?
问题描述:
1+1/(1+2)+1/(1+2+3)+1/(1+2+3+4)+……+1/(1+2+3+4……+10)等于几?
答
1+1/(1+2)+1/(1+2+3)+1/(1+2+3+4)+……+1/(1+2+3+4……+10)
=1/ (1*2 /2) +1/ (2*3 /2) +1/ (3*4 /2) +1/ (4*5 /2)+……+1/ (10*11 /2)
=2/ 1*2 +2/ 2*3 +2/ 3*4 +2/ 4*5 +……+ 2/ 10*11
=2 * (1/ 1*2 +1/ 2*3 +1/ 3*4 +1/ 4*5+……+1/ 10*11)
=2 * (1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+……+1/10-1/11)
=2 * (1-1/11)
=2 * 10/11
=20/11
就是自然数列求和及拆项法的组合.