Question

tính:

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+....+\frac{2}{20.21}\)

giúp mik nha,ai làm nhanh và rõ ràng chinh xác thì mik sẽ k 3 cái luôn nha!!

Answer 1

2/1.2*2/20.21

Answer 2

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{20.21}\)

\(=2\left(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{20.21}\right)\)

\(=2\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{20}-\frac{1}{21}\right)\)

\(=2\left(1-\frac{1}{21}\right)=2.\frac{20}{21}=\frac{40}{21}\)

Answer 3

=2.(1-1/2+1/2-1/3+1/3-1/4+...+1/20-1/21) =2.(1-1/21) =2.20/21 =40/21 k mik nha

Answer 4

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{20.21}\)

\(=2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{20}-\frac{1}{21}\right)\)

\(=2.\left(1-\frac{1}{21}\right)=2\cdot\frac{20}{21}=\frac{40}{21}\)

Answer 5

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+...+\frac{2}{20.21}\)

\(=2.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{20.21}\right)\)

\(=2.\left(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{20}-\frac{1}{21}\right)\)

\(=2.\left(\frac{1}{1}-\frac{1}{21}\right)\)

\(=\frac{2.20}{21}=\frac{40}{21}\)

Answer 6

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+.....+\frac{2}{20.21}\)

\(=2.\left(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{20.21}\right)\)

\(=2.\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{20}-\frac{1}{21}\right)\)

\(=2.\left(1-\frac{1}{21}\right)\)

\(=2.\frac{20}{21}\)

\(=\frac{40}{21}\)

Answer 7

\(\frac{2}{1.2}+\frac{2}{2.3}+\frac{2}{3.4}+..+\frac{2}{20.21}\)

=\(2\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{20}-\frac{1}{21}\right)\)

=\(2\left(1-\frac{1}{21}\right)\)

=2.\(\frac{20}{21}\)

=\(\frac{40}{21}\)

k mik nha bn !