作业帮1/1*3+1/2*4+1/3*5+…1/18*20=?
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1/1*3+1/2*4+1/3*5+…1/18*20=?
1/1*3+1/2*4+1/3*5+…1/18*20=?
1/1*3+1/2*4+1/3*5+…1/18*20=?
原式=(1/2)[(1/1)-(1/3)+(1/2)-(1/4)+..+(1/17)-(1/19)+(1/18)-(1/20)]
=(1/2)[1-(1/19)+(1/2)-(1/20)]
=(1/2)[(18/19)+(9/20)]
=9/19+9/40
=118/760=59/380
1/1*3+1/2*4+1/3*5+…1/18*20
=1/2(1-1/3+1/2-1/4+1/3-1/5+......+1/18-1/20)
=1/2(1-1/19+1/2-1/20)
=9/19+9/40
=531/760
1/1*3+1/2*4+1/3*5+…1/18*20
=1/2×（1-1/3+1/2-1/4+1/3-1/5+..-..+1/18-1/20)
=1/2×（1+1/2-1/19-1/20)
=531/760
1/1*3+1/2*4+1/3*5+…1/18*20
=1/2(1-1/3)+1/2(1/2-1/4)+1/2(1/3-1/5)+...+1/2(1/18-1/20)
=1/2(1-1/3+1/2-1/4+1/3-1/5+..+1/16-1/18+1/17-1/19+1/18-1/20)
=1/2(1+1/2-1/19-1/20)=531/760
你那个是乘还是乘方啊
1/1*3+1/2*4+1/3*5+…1/18*20
=1/2(1-1/3+1/2-1/4+1/3-1/5+......+1/18-1/20)
=1/2(1-1/19+1/2-1/20)
=9/19+9/40
=531/760