Question

A=\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+\cdot\cdot\cdot+\frac{1}{18\cdot19}+\frac{1}{19\cdot20}\)

Answer 1

A=1/1x2 +1/2x3 +... +1/18x19 + 1/19x20

Nhận xét 1/1x2 = 1/1 -1/2 ; 1/2x3=1/2-1/3; ... ;1/18x19=1/18-1/19 ; 1/19x20=1/19-1/20

ta có A=1/1 - 1/2 + +1/2 -1/3+1/3-   +1/18-1/19+1/19-1/20

         A=1/1 - 1/20 

         A=20/20 - 1/20

         A=(20-1)/20

         A=19/20

                   Vậy A=19/20

Answer 2

A =\(\frac{1}{1.2}\)+\(\frac{1}{2.3}\)+\(\frac{1}{3.4}\)+ ...+\(\frac{1}{18.19}\)+\(\frac{1}{19.20}\)

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

A = 1 - \(\frac{1}{20}\)( Vì đã triệt tiêu )

A = \(\frac{19}{20}\)

Answer 3

a=(1/1-1/2) + (1/2 +1/3) +(1/3+1/4)+........+(1/2016+1/2017)

a=1/1-1/2+1/2-1/3+1/3-1/4+ ........+2016-2017

a=1/1+1/2017 =2017-1/2017

a=2016/2017

chac chan 100% do ban .tk mk nha

Answer 4

=19/20

Answer 5

=2-1/1.2+3-2/2.3+4-3/3.4+...+19-18/18.19+20-19/19.20

=2/1.2-1/1.2+2/2.3-3/2.3+.....+20/19.20-19/19.20

=1-1/2+1/2-1/3+1/3-1/4+...+1/19-1/20

=1-1/20

=19/20

Answer 6

A=19/20

don gian ma

Answer 7

<br class="Apple-interchange-newline"><div id="inner-editor"></div>11.2 +12.3 +13.4 + ...+118.19 +119.20 

A = 1 - 12 +12 - 13 +13 - 14 +....+118 - 119 + 119 - 120 

A = 1 - 120 ( Vì đã triệt tiêu )

A = 

Answer 8

\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{18\cdot19}+\frac{1}{19\cdot20}\)

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

\(=\frac{1}{1}-\frac{1}{20}\)( đã xóa những phân số giống nhau )

\(=\frac{19}{20}\)

Answer 9

Ta có:

\(A=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{18.19}+\frac{1}{19.20}\)

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

\(\Rightarrow A=\frac{1}{1}-\frac{1}{20}=\frac{19}{20}\)

Answer 10

đụ mạ mi