A = 15x17x19x21x23x25x27x29x...x2011x2013

A = \(\overline{..5}\)

Vì A là tích của thừa số số có tận cùng là 5 với các thừa số lẻ

 \(\dfrac{2}{3}\times\dfrac{3}{4}\times\dfrac{4}{5}\) \(=\dfrac{2\times3\times4}{3\times4\times5}\) \(=\) \(\dfrac{2}{5}\)

\(\dfrac{2}{3}x\dfrac{3}{4}x\dfrac{4}{5}=\dfrac{2}{5}\)

Bài 1:

a) \(x.\dfrac{3}{4}=\dfrac{9}{14}\)

\(\Rightarrow x=\dfrac{9}{14}:\dfrac{3}{4}\)

\(\Rightarrow x=\dfrac{6}{7}\)

b) \(x:\dfrac{5}{9}=\dfrac{3}{10}\)

\(\Rightarrow x=\dfrac{3}{10}.\dfrac{5}{9}\)

\(\Rightarrow x=\dfrac{1}{6}\)

help me

\(...=\dfrac{4}{5}x\left(\dfrac{2}{3}+\dfrac{1}{3}\right)\)

\(=\dfrac{4}{5}x1\)

\(=\dfrac{4}{5}\)

\(\dfrac{2}{3}\times\dfrac{4}{5}+\dfrac{1}{3}\times\dfrac{4}{5}\\ =\dfrac{4}{5}\times\left(\dfrac{2}{3}+\dfrac{1}{3}\right)\\ =\dfrac{4}{5}\times1\\ =\dfrac{4}{5}\)

\(\dfrac{1}{7}+\dfrac{2}{7}+\dfrac{3}{7}+\dfrac{4}{7}+\dfrac{5}{7}+\dfrac{6}{7}\)

\(=\dfrac{1+2+3+4+5+6}{7}=3\)

\(\dfrac{1}{7}+\dfrac{2}{7}+\dfrac{3}{7}+\dfrac{4}{7}+\dfrac{5}{7}+\dfrac{6}{7}\)

\(=\left(\dfrac{1}{7}+\dfrac{6}{7}\right)+\left(\dfrac{2}{7}+\dfrac{5}{7}\right)+\left(\dfrac{3}{7}+\dfrac{4}{7}\right)\)

\(=1+1+1\)

\(=3\)

h

= (198065 x 345789) x 0

= 0

\(\dfrac{7}{12}< \dfrac{x}{24}< \dfrac{2}{3}\)

Ta có:

\(\dfrac{7}{12}=\dfrac{7x2}{12x2}=\dfrac{14}{24}\)\(;\dfrac{2}{3}=\dfrac{2x8}{3x8}=\dfrac{16}{24}\)

\(\Rightarrow x=15\)

\(\dfrac{7}{12}< \dfrac{x}{24}< \dfrac{2}{3}\)

\(\Rightarrow\dfrac{7\times2}{12\times2}< \dfrac{x}{24}< \dfrac{2\times8}{3\times8}\)

\(\Rightarrow\dfrac{14}{24}< \dfrac{x}{24}< \dfrac{16}{24}\)

\(\Rightarrow14< x< 16\)

P = (a + b) x 2

S = a x h

Số kg thóc mẹ thu được là:
\(\text{120 : 2 x 5 = 300 (kg)}\)
\(\text{ Đổi : 300kg = 3 tạ}\)
Đáp số: 3 tạ thóc

Mẹ thu được số tạ thóc là:

(120 : 2) x 5 = 300 (kg)

                    = 3 tạ

Đáp số: 3 tạ thóc

\(x\times1515+x\times4545=2\)

\(x\times\left(1515+4545\right)=2\)

\(x\times6060=2\)

\(x=\dfrac{2}{6060}=\dfrac{1}{3030}\)

\(x.1515+x.4545=2\)

\(\Rightarrow x.\left(1515+4545\right)=2\)

\(\Rightarrow x.6060=2\)

\(\Rightarrow x=2:6060\)

\(\Rightarrow x=\dfrac{1}{3030}\)