A=2/6+2/12+...+2/90
=2/2x3+2/3x4+..+2/9x10
=2(1/2-1/3+1/3-1/4+...+1/9-1/10)
=2(1/2-1/10)=2x2/5=4/5.
A = \(\dfrac{2}{6}\) + \(\dfrac{2}{12}\) + \(\dfrac{2}{20}\) +...+ \(\dfrac{2}{90}\)
A = \(2\times\) ( \(\dfrac{1}{2\times3}\) + \(\dfrac{1}{3\times4}\)+...+\(\dfrac{1}{9\times10}\))
A = 2 \(\times\) ( \(\dfrac{1}{2}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{4}\)+...+ \(\dfrac{1}{9}\) - \(\dfrac{1}{10}\))
A = 2 \(\times\)( \(\dfrac{1}{2}\) - \(\dfrac{1}{10}\))
A = 2 \(\times\) \(\dfrac{2}{5}\)
A = \(\dfrac{4}{5}\)
A = 2/(2.3) + 2/(3.4) + 2/(4.5) + ... + 2/(9.10)
= 2.(1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/9 - 1/10)
= 2.(1/2 - 1/10)
= 2.2/5
= 4/5
\(A=\dfrac{2}{6}+\dfrac{2}{12}+\dfrac{2}{20}+...+\dfrac{2}{90}\\ \\ A=\dfrac{2}{2.3}+\dfrac{2}{3.4}+\dfrac{2}{4.5}+...+\dfrac{2}{9.10}\\ \\ A=2.\left(\dfrac{1}{2.3}+\dfrac{1}{3.4}+\dfrac{1}{4.5}+...+\dfrac{1}{9.10}\right)\\ \\ A=2.\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\\ \\ A=2.\left(\dfrac{1}{2}-\dfrac{1}{10}\right)\\ \\ A=2.\left(\dfrac{5}{10}-\dfrac{1}{10}\right)\\ \\ A=2.\dfrac{2}{5}=\dfrac{4}{5}\\\)
. LÀ x
A=2/6+2/12+...+2/90
=2/2x3+2/3x4+..+2/9x10
=2(1/2-1/3+1/3-1/4+...+1/9-1/10)
=2(1/2-1/10)=2x2/5=4/5.
2/6 + 2/12 + 2/20 +...+ 2/90
= 2.(1/6 + 1/12 + 1/20 +...+ 1/90)
= 2.(1/2 - 1/3 + 1/3 - 1/4 + 1/4 -1/5 +...+ 1/9 - 1/10)
= 2.(1/2 - 1/10)
= 2. 2/5
= 4/5
* Dấu . là dấu nhân đó nha
* 1/2 - 1/3 = 1/6, tương tự với 1/12,1/20,...,1/90
