3*5/1+5*7/1+7*9/1+9*11/1+.+37*39/1=( )

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3*5/1+5*7/1+7*9/1+9*11/1+.+37*39/1=( )

3*5/1+5*7/1+7*9/1+9*11/1+.+37*39/1=( )
3*5/1+5*7/1+7*9/1+9*11/1+.+37*39/1=( )

3*5/1+5*7/1+7*9/1+9*11/1+.+37*39/1=( )
3*5/1=1/2(1/3-1/5)
5*7/1=1/2(1/5-1/7)
.
所以原式=1/2(1/3-1/5+1/5-1/7+1/7-1/9+.+1/37-1/39)
=1/2(1/3-1/39)
=1/2(13/39-1/39)
=1/2*12/39
=6/39

511=9
9u

2/13

1/2(1/3-1/5+1/5-1/7+1/7-1/9+。。。+1/37-1/39)
=1/2(1/3-1/39)
=1/2(13/39-1/39)
=1/2*12/39
=6/39