Question

Tính nhanh: \(\frac{1}{2x3}+\frac{1}{3x4}+\frac{1}{4x5}+...\frac{1}{18x19}+\frac{1}{19x20}\)

Answer 1

\(\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\frac{1}{4\cdot5}+...+\frac{1}{19\cdot20}\)

\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{19}-\frac{1}{20}\)

\(=\frac{1}{2}-\frac{1}{20}\)

\(=\frac{9}{20}\)

Answer 2

\(\frac{1}{2x3}\)+ \(\frac{1}{3x4}\)+ \(\frac{1}{4x5}\)+ ... + \(\frac{1}{18x19}\)+ \(\frac{1}{19x20}\)

= \(\frac{1}{2}\)- \(\frac{1}{3}\)+ \(\frac{1}{3}\)- \(\frac{1}{4}\)+ \(\frac{1}{4}\)- \(\frac{1}{5}\)+ ... + \(\frac{1}{18}\)- \(\frac{1}{19}\)+ \(\frac{1}{19}\)- \(\frac{1}{20}\)

= \(\frac{1}{2}\)- \(\frac{1}{20}\)

= \(\frac{18}{40}\)= \(\frac{9}{20}\)

Answer 3

=1/2-1/3+1/3-1/4+...+1/18-1/19+1/19-1/20         K MIK NHA MOI NGUOI 

=1/2-1/20

=10/20-1/20

=9/20

Answer 4

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Answer 5

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