分子=1-1/2+1/3-1/4+1/5-1/6……+1/99-1/100
=1+1/2+1/3+1/4+1/5+1/6……+1/99+1/100-2*(1/2+1/4+1/6+.+1/100)
=1+1/2+1/3+1/4+1/5+1/6……+1/99+1/100-(1+1/2+1/3+.+1/50)
=1/51+1/52+1/53+.+1/100
分母=1/102+1/104+1/106+.+1/200
=1/2*(1/51+1/52+1/53+.+1/100)
分子除分母=1/(1/2)=2