A = (2 + 2²) + (2³ + 2⁴ ) +…(2¹⁹⁹ + 2²⁰⁰)

A = 6 + 2² (2 + 2² ) +… + 2¹⁹⁸ (2 + 2²)

A = 6 + 2² (6 ) +… + 2¹⁹⁸ (6)

A = 6(1 + 2² +… + 2¹⁹⁸)

Vậy A chia hết cho 6

a) \(\left(\frac{2}{7}+\frac{3}{8}+\frac{12}{7}+\frac{5}{8}\right).\frac{19}{36}\)

\(=\left\{\left(\frac{2}{7}+\frac{12}{7}\right)+\left(\frac{3}{8}+\frac{5}{8}\right)\right\}.\frac{19}{36}\)

\(=\left(\frac{14}{7}+\frac{8}{8}\right).\frac{19}{36}\)

\(=\left(2+1\right).\frac{19}{36}\)

\(3.\frac{19}{36}=\frac{19}{12}\)

8/15 + 7/4 + 7/15 + 5/4

= 8/15 + 7/15 + 7/4 +5/4

= ( 8/15 + 7/15) + ( 7/4 + 5/4)

= 15/15 + 12/4

= 1 + 3

=4

=8/15+7/15+7/4+5/4

=1+3

=4

\(\dfrac{15}{8}\) : \(\dfrac{5}{20}\) - \(\dfrac{7}{8}\) : \(\dfrac{1}{4}\) 

= \(\dfrac{15}{8}\) : \(\dfrac{1}{4}\) - \(\dfrac{7}{8}\) : \(\dfrac{1}{4}\)

= ( \(\dfrac{15}{8}\) - \(\dfrac{7}{8}\)) : \(\dfrac{1}{4}\)

= \(\dfrac{8}{8}\) \(\times\) 4

= 4

\(\dfrac{9}{4}\times\dfrac{15}{11}+\dfrac{3}{4}:\dfrac{11}{7}-\dfrac{3}{4}\times\dfrac{8}{11}\)
\(=\dfrac{3}{4}\times3\times\dfrac{15}{11}+\dfrac{3}{4}\times\dfrac{7}{11}-\dfrac{3}{4}\times\dfrac{8}{11}\)
\(=\dfrac{3}{4}\times\dfrac{45}{11}+\dfrac{3}{4}\times\dfrac{7}{11}-\dfrac{3}{4}\times\dfrac{8}{11}\)
\(=\dfrac{3}{4}\times\left(\dfrac{45}{11}+\dfrac{7}{11}-\dfrac{8}{11}\right)\)
\(=\dfrac{3}{4}\times4\)
\(=3\)

`9/4xx15/11+3/4:11/7-3/4xx8/11`

`=3/4xx15/11xx3+3/4xx7/11-3/4xx8/11`

`=3/4xx45/11+3/4xx7/11-3/4xx8/11`

`=3/4xx(45/11+7/11-8/11)`

`=3/4xx4`

`=3`

\(a.60\times\left(\frac{7}{12}+\frac{4}{15}\right)\)

\(=60\times\left(\frac{35}{60}+\frac{16}{60}\right)\)

\(=60\times\frac{51}{60}\)

\(=51\)

\(b.\frac{1}{2}\times\frac{2}{3}\times\frac{3}{4}\times\frac{4}{5}\times\frac{5}{6}\times\frac{6}{7}\times\frac{7}{8}\times\frac{8}{9}\)

\(=\frac{1\times2\times3\times4\times5\times6\times7\times8}{2\times3\times4\times5\times6\times7\times8\times9}\)

\(=\frac{1}{9}\)

Bài làm

   \(\frac{7}{15}x\frac{3}{8}+\frac{7}{15}x\frac{5}{8}\)

= \(\frac{7}{15}x\left(\frac{3}{8}+\frac{5}{8}\right)\)

= \(\frac{7}{15}x\frac{8}{8}\)

= \(\frac{7}{15}x1\)

= \(\frac{7}{15}\)

# Chúc bạn học tốt #

\(\frac{7}{15}\cdot\frac{3}{8}+\frac{7}{15}\cdot\frac{5}{8}\)

\(=\frac{7}{15}\cdot(\frac{3}{8}+\frac{5}{8})\)

\(=\frac{7}{15}\cdot1=\frac{7}{15}\)

\(\frac{7}{15}\times\frac{3}{8}+\frac{7}{15}\times\frac{5}{8}\)

\(=\frac{7}{15}\times\left(\frac{3}{8}+\frac{5}{8}\right)\)

\(=\frac{7}{15}\times1\)

\(=\).....................

Tự làm tiếp nhé!

=15/17 x (4/7 + 3/7) - 1/3
=15/17 x 1 - 1/3
=15/17 x (3/3 - 1/3)
=15/17 x 2/3
=30/51