1乘3分之1加3乘5分之1加5乘7分之1…97乘99分之一=1/2×(1-1/3)+1/2×(1/3-1/5)+1/2×(1/5-1/7)+……+1/2×(1/97-1/99)=1/2×(1-1/3+1/3-1/5+1/5-1/7+……+1/97-1/99)=1/2×(1-1/99)=1/2×98/99=49/99...