405 j vậy ạ

Uhhhhhh,

     bn ơi,405 gì zị?

Số mét vải cần dùng để may 1 chiếc khăn là:

4:5=0,8(mét)

May 1 chiếc khăn hết số mét vải là:

     4 : 5 = 0,8 (m)

             Đ/S : 0,8 m vải 

                                      :>

Để may 1 chiếc khăn thì cần:

4:5=0,8(m)

a: \(\dfrac{4}{13}+\dfrac{-12}{39}=\dfrac{4}{13}-\dfrac{4}{13}=0\)

b: \(\dfrac{27}{23}-\dfrac{-5}{21}-\dfrac{4}{23}+\dfrac{16}{21}+\dfrac{1}{2}\)

\(=\left(\dfrac{27}{23}-\dfrac{4}{23}\right)+\left(\dfrac{5}{21}+\dfrac{16}{21}\right)+\dfrac{1}{2}\)

\(=1+1+\dfrac{1}{2}=\dfrac{5}{2}\)

c: \(\dfrac{-8}{9}+\dfrac{1}{9}\cdot\dfrac{2}{9}+\dfrac{1}{9}\cdot\dfrac{7}{9}\)

\(=\dfrac{-8}{9}+\dfrac{1}{9}\left(\dfrac{2}{9}+\dfrac{7}{9}\right)\)

\(=\dfrac{-8}{9}+\dfrac{1}{9}=\dfrac{-7}{9}\)

d: \(\dfrac{2}{\left(-3\right)^2}+\dfrac{5}{-12}-\dfrac{-3}{4}\)

\(=\dfrac{2}{9}-\dfrac{5}{12}+\dfrac{3}{4}\)

\(=\dfrac{8}{36}-\dfrac{15}{36}+\dfrac{27}{36}=\dfrac{19}{36}\)

- Với \(p=3\Rightarrow8p-1=8.3-1=23\) là số nguyên tố và \(8p+1=25\) là hợp số

- Với \(p\ne3\Rightarrow p\) không chia hết cho 3

\(\Rightarrow p=3k+1\) hoặc \(p=3k+2\)

Với \(p=3k+1\Rightarrow8p+1=8\left(3k+1\right)+1=24k+9=3\left(8k+3\right)\) chia hết cho 3 \(\Rightarrow\) là hợp số

Với \(p=3k+2\Rightarrow8p-1=8\left(3k+2\right)-1=24k+15=3\left(8k+5\right)\) chia hết cho 3 \(\Rightarrow\) là hợp số

Vậy số còn lại luôn là hợp số

\(ac=b^2\)

=>\(\dfrac{a}{b}=\dfrac{b}{c}\)

\(bd=c^2\)

=>\(\dfrac{b}{c}=\dfrac{c}{d}\)

=>\(\dfrac{a}{b}=\dfrac{b}{c}=\dfrac{c}{d}=k\)

=>\(\left\{{}\begin{matrix}c=dk\\b=ck=dk\cdot k=dk^2\\a=bk=dk^3\end{matrix}\right.\)

\(\left(\dfrac{2a+3b-c}{2b+3c-d}\right)^3=\left(\dfrac{2\cdot dk^3+3\cdot dk^2-dk}{2\cdot dk^2+3\cdot dk-d}\right)^3\)

\(=\left(\dfrac{dk\left(2k^2+3k-1\right)}{d\left(2k^2+3k-1\right)}\right)^3=k^3\)

\(\dfrac{a}{d}=\dfrac{dk^3}{d}=k^3\)

Do đó: \(\dfrac{a}{d}=\left(\dfrac{2a+3b-c}{2b+3c-d}\right)^3\)

tốc độ đạp xe của Lan là:

200:10=20(km/h)

Vận tốc đạp xe của Lan là:

200:10=20(km/h)

Bài 2:

a: \(\dfrac{7}{8}+x=\dfrac{3}{5}\)

=>\(x=\dfrac{3}{5}-\dfrac{7}{8}=\dfrac{24-35}{40}=\dfrac{-11}{40}\)

b: \(\dfrac{17}{2}:x=5\)

=>\(x=\dfrac{17}{2}:5\)

=>\(x=\dfrac{17}{2\cdot5}=\dfrac{17}{10}\)

c: \(x-\dfrac{3}{8}=2+\dfrac{1}{4}\)

=>\(x-\dfrac{3}{8}=\dfrac{9}{4}\)

=>\(x=\dfrac{9}{4}+\dfrac{3}{8}=\dfrac{18}{8}+\dfrac{3}{8}=\dfrac{21}{8}\)

d: \(\dfrac{1}{2}+\dfrac{3}{5}\left(x-2\right)=\dfrac{1}{5}\)

=>\(\dfrac{3}{5}\left(x-2\right)=\dfrac{1}{5}-\dfrac{1}{2}=\dfrac{-3}{10}\)

=>\(x-2=-\dfrac{1}{2}\)

=>\(x=2-\dfrac{1}{2}=\dfrac{3}{2}\)

Bài 1:

a: \(\dfrac{-3}{4}+\dfrac{1}{5}=\dfrac{-15}{20}+\dfrac{4}{20}=\dfrac{-15+4}{20}=\dfrac{-11}{20}\)

b: \(\dfrac{-2}{5}-\dfrac{1}{3}=\dfrac{-6}{15}-\dfrac{5}{15}=\dfrac{-6-5}{15}=\dfrac{-11}{15}\)

c: \(\dfrac{3}{7}\cdot\dfrac{2}{5}-\dfrac{2}{5}=\dfrac{2}{5}\left(\dfrac{3}{7}-1\right)=\dfrac{2}{5}\cdot\dfrac{-4}{7}=\dfrac{-8}{35}\)

d: \(\dfrac{1}{4}+\dfrac{3}{4}\left(\dfrac{2}{3}-\dfrac{1}{2}\right)\)

\(=\dfrac{1}{4}+\dfrac{3}{4}\cdot\dfrac{4-3}{6}\)

\(=\dfrac{1}{4}+\dfrac{3}{4}\cdot\dfrac{1}{6}=\dfrac{1}{4}+\dfrac{1}{8}=\dfrac{3}{8}\)

e: \(\dfrac{7}{2}\cdot\dfrac{8}{13}+\dfrac{8}{13}\cdot\dfrac{-5}{12}+\dfrac{8}{13}\)

\(=\dfrac{8}{13}\left(\dfrac{7}{2}-\dfrac{5}{2}+1\right)\)

\(=\dfrac{8}{13}\cdot2=\dfrac{16}{13}\)

f: \(1+\dfrac{1}{8}+\dfrac{1}{24}+\dfrac{1}{48}+\dfrac{1}{80}+\dfrac{1}{120}\)

\(=1+\dfrac{1}{2\cdot4}+\dfrac{1}{4\cdot6}+\dfrac{1}{6\cdot8}+\dfrac{1}{8\cdot10}+\dfrac{1}{10\cdot12}\)

\(=1+\dfrac{1}{2}\left(\dfrac{2}{2\cdot4}+\dfrac{2}{4\cdot6}+...+\dfrac{2}{10\cdot12}\right)\)

\(=1+\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+...+\dfrac{1}{10}-\dfrac{1}{12}\right)\)

\(=1+\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{12}\right)\)

\(=1+\dfrac{1}{2}\cdot\dfrac{5}{12}=1+\dfrac{5}{24}=\dfrac{29}{24}\)

Tổng số phần bằng nhau là 2+3=5(phần)

Số gạo tẻ còn lại là \(135\cdot\dfrac{3}{5}=81\left(kg\right)\)

Số gạo nếp còn lại là 81:1,5=54(kg)

Số gạo tẻ ban đầu là \(81:\left(1-\dfrac{2}{5}\right)=81:\dfrac{3}{5}=135\left(kg\right)\)

Số gạo nếp ban đầu là:

\(54:\left(1-\dfrac{2}{3}\right)=54:\dfrac{1}{3}=54\cdot3=162\left(kg\right)\)

-1/4 + 2/5 . x = 4/15

2/5 . x = 4/15 + 1/4

2/5 . x = 31/60

x = 31/60 : 2/5

x = 31/24

-1/4+2/5.x=4/15

        2/5.x=4/15-1/4

        2/5.x=1/60

              X=1/60:2/5

              X=1/60.5/2

             X= 1/24

-1/4+2/5.x=4/15