1/1*3+1/3*5+.1/(2n-1)(2n+1)

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1/1*3+1/3*5+.1/(2n-1)(2n+1)
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1/1*3+1/3*5+.1/(2n-1)(2n+1)
1/1*3+1/3*5+.1/(2n-1)(2n+1)

1/1*3+1/3*5+.1/(2n-1)(2n+1)
=1/2*[1-1/3+1/3-1/3+……+1/(2n-1)-1/(2n+1)]
=1/2*[1-1/(2n+1)]
=n/(2n+1)

原式=1/2[1/1-1/3+1/3-1/5...........+1/(2n-1)-1/(2n+1)]
=1/2-1/2(2n+1)

½ [2/1*3+2/3*5+……+2/(2n-1)(2n+1)]
=½ {(3-1)/1*3 + (5-3)/3*5+……+[(2n+1)-(2n-1)]/(2n-1)(2n+1)]}
=½ [1-1/3 +1/3-1/5+……+1/(2n-1)-1/(2n+1)]
=½ [1-1/(2n+1)]
=½* 2n/(2n+1)
=n/(2n+1)