Question

1/3+1/6+1/10+......+2/Xx(X+1)=2011/2013

Tìm x

Answer 1

1/2x3/2+1/3x4/2+1/4x5/2+1/5x6/2+.......+2/Xx(X+1)=2011/2013

2/2x3+2/3x4+2/4x5+2/5x6+.....+2/Xx(X+1)=2011/2013

2x(1/2x3+1/3x4+1/4x5+1/5x6+....+1/Xx(x+1)=2011/2013

1/2x3+1/3x4+1/4x5+1/5x6+....+1/Xx(X+1)=2011/4026

1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+.....+ 1/x-1/x+1=2011/4026

1/2-1/x+1=2011/4026

1/x+1=1/2-2011/4026

1/x+1=1/2013

Suy ra x=2012

Answer 2

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