a=1/2+1/6+1/12+1/20+1/30+1/n
a=1/1-1/2+1/2-...+1/a+1
a=1+1/a-1=39
=>a=40
vay n=39X40=1560
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cho a =1/2+1/6+1/12+1/20+1/30+.....+1/n biet a +39/40
A= 1/2+1/6+1/12+20+1/30+....+1/n
biet A=39/40 tim n
cho a = 1/2+1/6+1/12+1/20+1/30+..........+1/82 biet a la 39/40 tim a
A=1/2+1/6+1/12+1/20+1/30+...+1/n Biết A=39/40 Tìm n
Cho A = 1/2 + 1/6 + 1/12 + 1/20 +1/30 + ....... + 1/n . Biết A =39/40.Tìm n.
a= 1/2 + 1/6 +1/12 + 1/20 +1/30 + .... + 1/n. tìm n biết a= 39/40
a= 1/2 + 1/6 +1/12 + 1/20 +1/30 + .... + 1/n. tìm n biết a= 39/40
Cho A= 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + ........ + 1/n = 39/40
A=1/2+1/6+1/12+1/20+1/30+...+1/n
Biết A=39/40 tìm n.