Question

[Trương Đại Dương]

Tính nhanh: 4/9+4/45+4/105+4/189

Answer 1

[Bùi Như Quỳnh]

A = \(\dfrac{4}{9}\)+ \(\dfrac{4}{45}\)+ \(\dfrac{4}{105}\)+ \(\dfrac{4}{189}\)

A = \(\dfrac{2}{3}\)( \(\dfrac{2}{3}\)+ \(\dfrac{2}{15}\)+ \(\dfrac{2}{35}\)+ \(\dfrac{2}{63}\))

A = \(\dfrac{2}{3}\)(\(\dfrac{2}{1.3}\)+ \(\dfrac{2}{3.5}\)+ \(\dfrac{2}{5.7}\)+ \(\dfrac{2}{7.9}\))

A = \(\dfrac{2}{3}\)( 1- \(\dfrac{1}{3}\)+ \(\dfrac{1}{3}\)- \(\dfrac{1}{5}\)+ \(\dfrac{1}{5}\)- \(\dfrac{1}{7}\)+ \(\dfrac{1}{7}\)- \(\dfrac{1}{9}\))

A = \(\dfrac{2}{3}\)(1 - \(\dfrac{1}{9}\))

A = \(\dfrac{2}{3}\). \(\dfrac{8}{9}\)

A = \(\dfrac{16}{27}\)

Answer 2

[Bùi Như Quỳnh]

dấu . là dấu nhân bạn nhé