RĂSPUNS:
1/2+1/6+1/12+1/20+1/30+...+1/420= =1/(1•2)+1/(2•3)+1/(3•4)+1/(4•5)+1/(5•6)+...+1/(20•21) =(1/1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+...+(1/20-1/21)
=1/1-1/21
=20/21
Poti utiliza formula : 1/(1•2)+1/(2•3)+1/(3•4)+1/(4•5)+...+1/[n•(n+1)]=n/(n+1)
Cu drag,
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