1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5).1/(19×20)等于多少?

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1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5).1/(19×20)等于多少?1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5).1/(19×20)等于多少?1/(1×2)+

1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5).1/(19×20)等于多少?
1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5).1/(19×20)等于多少?

1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5).1/(19×20)等于多少?
1/1X2=1-1/2,以此类推.所以原式=1-1/2+1/2-1/3+1/3-1/4+……+1/19-1/20
=1-1/20
=19/20

=1-1/2+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+……(1/19-1/20)=1-1/20=19/20

原式=1-1/2+1/2-1/3+1/3-1/4+……+1/19-1/20
=1-1/20
=19/20