裂项
2/(1×3)=1-1/3
2/(3×5)=1/3-1/5
.....
2/(89×91)=1/89-1/91
原式=1-1/3+1/3-1/5+...+1/89-1/91
=1-1/91
=90/91
=1-1/3+1/3-1/5+1/5-1/7+……+1/891/91
=1-1/91
=90/91
解：原式=2（2/1-2/3+2/3-2/5+……+2/89-2/91)
=2(2/1-2/91)
=186/91
2/1*3+2/3*5+2/5*7......+2/89*91
=(1-1/3)+1/3-1/5+1/5-1/7+...+1/89-1/91
=1-1/91
=90/91
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