Răspuns:
S2=1/1*3+1/2*4+1/3*5+...+1/98*100
1/1*3=1/2(1-1/3)
1/2*4=1/2(1/2-1/4)
1/3*5=1/2(1/3-1/5)
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1/98*100=1/2(1/98-1/100) adui egalitatile membru cu membru
S2=1/2(1-1/3+1/2-1/4+1/3-1/5+....+1/98-1/100)=
1/2(1+1/2-1/100)=
1/2(3/2-1/100)=
1/2*(150-1)/100=
100/2*149=50/149
Explicație pas cu pas: