1/（1×3）+1/（2×4）+1/（3×5）+……+1/（9×11）=?
问题描述：
1/（1×3）+1/（2×4）+1/（3×5）+……+1/（9×11）=?
答
原式=(1-1/3+1/2-1/4+1/3-1/5+……+1/9-1/11)/2 =(1+1/2-1/10-1/11)/2 =(3/2-21/110)/2 =72/110 =36/55