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Question

Tìm x, biết:

a) 1/5.8 + 1/8.11 +1/11.14 +....+ 1/x(x+3) = 101/1540

b) 1+1/3+1/6+1/10+.....+1/x.(x+1):2 =1/1991/1993

Answer 1

Bài làm (sưu tầm)

a) 1/5.8+1/8.11+1/11.14+......+1/x.(x+3)=101/1540

1/3.3.[1/5.8+1/8,11+1/11.14+......+1/x.(x+3)=101/1540

1/3.[3/5.8+3/8.11+3/11.14+........+3/x.(x+3)]=101/1540

1/3.[1/5-1/8+1/8-1/11+1/11-1/14+....+1/x-1/x+3=101/1540

1/3.[1/5-1/x+3]=101/1540

1/5-1/x+3=101/1540.3

1/5-1/x+3=303/1540

1/x+3=1/3-303/1540=1/308

=>x+3=308    =>x=305

              Vậy x=305