1*2*3/1+2*3*4/1+...+9*10*11/1怎样简算?
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1*2*3/1+2*3*4/1+...+9*10*11/1怎样简算?
1 1 1
------ + -----+···+ -------等于多少?要简算.
1*2*3 2*3*4 9*10*11
1 1 1
------ + -----+···+ -------等于多少?要简算.
1*2*3 2*3*4 9*10*11
原式=(1/1)*[1/(2*3)]+(1/2)*[1/()3*4]+.+(1/9)[1/(10*11)]
=(1/1)*(1/2-1/3)+(1/2)*(1/3-1/4)+.+(1/9)(1/10-1/11)
=1/(1*2)-1/(1*3)+1/(2*3)-1/(2*4)+.+1/(9*10)-1/(9*11)
=[1/(1*2)+1/(2*3)+.+1/(9*10)]-[1/(1*3)+1/(2*4)+.+1/(9*11)]
=(1-1/2+1/2-1/3+.1/9-1/10)-(1/2)*[2/(1*3)+2/(2*4)+.+2/(9*11)]
=1-1/10-(1/2)*[1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+.+1/8-1/10+1/9-1/11]
=1-1/10-(1/2)*(1+1/2-1/10-1/11)
=1-1/10-1/2-1/4+1/20+1/22
=27/110
=(1/1)*(1/2-1/3)+(1/2)*(1/3-1/4)+.+(1/9)(1/10-1/11)
=1/(1*2)-1/(1*3)+1/(2*3)-1/(2*4)+.+1/(9*10)-1/(9*11)
=[1/(1*2)+1/(2*3)+.+1/(9*10)]-[1/(1*3)+1/(2*4)+.+1/(9*11)]
=(1-1/2+1/2-1/3+.1/9-1/10)-(1/2)*[2/(1*3)+2/(2*4)+.+2/(9*11)]
=1-1/10-(1/2)*[1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+.+1/8-1/10+1/9-1/11]
=1-1/10-(1/2)*(1+1/2-1/10-1/11)
=1-1/10-1/2-1/4+1/20+1/22
=27/110
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