Question

Tính tổng A=\(\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\).

Answer 1

\(A=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\)

\(A=2\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{240}\right)\)

\(A=2\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{15.16}\right)\)

\(A=2\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{15}-\frac{1}{16}\right)\)

\(A=2.\frac{3}{16}\)

\(A=\frac{3}{8}\)

Answer 2

A=2/20+2/30+2/42+...+2/240

=>A/2=1/20+1/30+1/42+...+1/240

=>A/2=1/(4*5)+1/(5*6)+1/(6*7)+...+1/(15*16)

=>A/2=1/4-1/5+1/5-1/5+1/6-1/7+...+1/15-1/16

=>A/2=1/4-1/16

=>A/2=3/16

=>A=3/8

Answer 3

A= 2/4.5+2/5.6+2/6.7+...+2/15.16

A=2.[1/4.5+1/5.6+1/6.7+...+1/15.16]

A=2.[1/4-1/5+1/5-1/6+1/6-1/7+...+/15-1/16]

A=2.[1/4-1/16]

A=2.3/16

A=3/8

Answer 4

A =( \(\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}\) ) : 2 x2

A =(\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{240}\)) \(\) x2

A =( \(\frac{1}{4x5}+\frac{1}{5x6}+\frac{1}{6x7}+...+\frac{1}{15x16}\) ) x 2 

A =(\(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}+\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{15}-\frac{1}{16}\) ) x2

A =( \(\frac{1}{4}-\frac{1}{16}\) ) x2 

A = \(\frac{3}{16}\) x 2

A = \(\frac{3}{8}\)

k nha Đức Anh Sociu