等差数列可写成2,4,6,8,10.
所以所求为1/2+1/6+1/12+1/20.
1/1*2+1/2*3+1/3*4+1/4*5.
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/n-1/(n+1)
=1-1/(n+1)