A = 1/1.2 + 1/2.3 +.......+1/a(a + 1)
A = 1/1 - 1/2 + 1/2 - ..... - 1/a+1
A= 1 - 1/a+ 1 = 39/40
1/a + 1 = 1/4 0 => a + 1= 40 => a= 39
Vậy n= 39 x 40 = 1560
a=1/2+1/6+1/12+1/20+1/30+...1/n
tim n biet a=39/40
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 biet 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.
