2/11*13+2/13*15+2/15*17+2/17*19+2/19*21+1/21
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![2/11*13+2/13*15+2/15*17+2/17*19+2/19*21+1/21](/uploads/image/z/30843-27-3.jpg?t=2%2F11%2A13%2B2%2F13%2A15%2B2%2F15%2A17%2B2%2F17%2A19%2B2%2F19%2A21%2B1%2F21)
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2/11*13+2/13*15+2/15*17+2/17*19+2/19*21+1/21
2/11*13+2/13*15+2/15*17+2/17*19+2/19*21+1/21
2/11*13+2/13*15+2/15*17+2/17*19+2/19*21+1/21
用裂项法
2/11*13+2/13*15+2/15*17+2/17*19+2/19*21+1/21
=1/11-1/13+1/13-1/15+...+1/19-1/21+1/21
=1/11
2/11*13=1/11 - 1/13
2/13*15=1/13 - 1/15
2/15*17=1/15 - 1/17
2/17*19=1/17 - 1/19
2/19*21=1/19 - 1/21
1/21=1/21
原式=1/11