Theo bài ra ta có : (x-2).4=3.2x                                                                                                                                                   4x-8  =6x                                                                                                                                                       4x-6x=8                                                                                                                                                            -2x=8                                                                                                                                                               x=-4                                                                                                                            

Gọi thời gian làm riêng để hoàn thành công việc tổ 1 ; 2 lần lượt là a ; b ( a ; b > 0 ) 

Theo bài ra ta có hệ \(\left\{{}\begin{matrix}\dfrac{1}{a}+\dfrac{1}{b}=\dfrac{1}{12}\\\dfrac{9}{a}+\dfrac{1}{2}.\dfrac{10}{b}+\dfrac{9}{b}=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\dfrac{1}{a}=\dfrac{1}{30}\\\dfrac{1}{b}=\dfrac{1}{20}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=30\\b=20\end{matrix}\right.\)(tm)

Vậy ... 

Đặt A=\(\dfrac{1}{11}\)+\(\dfrac{1}{12}\)+\(\dfrac{1}{13}\)+...\(\dfrac{1}{69}\)+\(\dfrac{1}{70}\)

         =(\(\dfrac{1}{11}\)+\(\dfrac{1}{12}\)+\(\dfrac{1}{13}\)+...\(\dfrac{1}{19}\)+\(\dfrac{1}{20}\))+(\(\dfrac{1}{21}\)+\(\dfrac{1}{22}\)+\(\dfrac{1}{23}\)+...+\(\dfrac{1}{29}\)+\(\dfrac{1}{30}\))+(\(\dfrac{1}{31}\)+\(\dfrac{1}{32}\)+\(\dfrac{1}{33}\)+...+\(\dfrac{1}{59}\)+\(\dfrac{1}{60}\))+...+\(\dfrac{1}{70}\)

Nhận xét:

\(\dfrac{1}{11}\)+\(\dfrac{1}{12}\)+\(\dfrac{1}{13}\)+...+\(\dfrac{1}{19}\)+\(\dfrac{1}{20}\)>\(\dfrac{1}{20}\)+\(\dfrac{1}{20}\)+...+\(\dfrac{1}{20}\)=\(\dfrac{10}{20}\)=\(\dfrac{1}{2}\)

\(\dfrac{1}{21}\)+\(\dfrac{1}{22}\)+\(\dfrac{1}{23}\)+...+\(\dfrac{1}{29}\)+\(\dfrac{1}{30}\)>\(\dfrac{1}{30}\)+\(\dfrac{1}{30}\)+...+\(\dfrac{1}{30}\)=\(\dfrac{10}{30}\)=\(\dfrac{1}{3}\)

\(\dfrac{1}{31}\)+\(\dfrac{1}{32}\)+\(\dfrac{1}{33}\)+...+\(\dfrac{1}{59}\)+\(\dfrac{1}{60}\)>\(\dfrac{1}{60}\)+\(\dfrac{1}{60}\)+...+\(\dfrac{1}{60}\)=\(\dfrac{30}{60}\)=\(\dfrac{1}{2}\)

=>A>\(\dfrac{1}{2}\)+\(\dfrac{1}{3}\)+\(\dfrac{1}{2}\)+\(\dfrac{1}{61}\)+...+\(\dfrac{1}{69}\)+\(\dfrac{1}{70}\)>\(\dfrac{1}{2}\)+\(\dfrac{1}{3}\)+\(\dfrac{1}{2}\)=\(\dfrac{4}{3}\)

=>A>\(\dfrac{4}{3}\)

Vậy: \(\dfrac{1}{11}\)+\(\dfrac{1}{12}\)+\(\dfrac{1}{13}\)+...+\(\dfrac{1}{69}\)+\(\dfrac{1}{70}\)>\(\dfrac{4}{3}\) (ĐPCM)

Thấy đúng cho 1 tick và 1 follow nha!

Chúc bạn học tốt!

=0,68 nha bạn

Trả lờ:

0,68

(x+1)+(x+2)+...x+28=155

=> x + 1 + x + 2 + ... + x + 28 =155

=> 28x + (1 + 2 + 3 + ... + 28) =155

=> 28x + 28.(28+1):2 = 155

=> 28x + 406 = 155

=> 28x = 155 - 406 = -251

=> x = -251/28

Ta có:

(x+1)+(x+2)+...x+28=155

<=> x + 1 + x + 2 + ... + x + 28 =155

<=> 28x + (1 + 2 + 3 + ... + 28) =155

<=> 28x + 28.(28+1):2 = 155

<=> 28x + 406 = 155

<=> 28x = 155 - 406 = -251

=> x = \(-\frac{251}{28}\)

hok tốt

ys em là j sao chị hk hỉu

5 ko phải là hỗn số

2/10 + 7 cũng vậy

cả 1/10

\(5\frac{2}{10}+7\frac{1}{10}=\frac{52}{10}+\frac{71}{10}=\frac{123}{10}\)

52/10 + 71/10 = 123/10

456/100 + 344/100 = 800/100 = 8

\(\frac{456}{100}\)+\(\frac{344}{100}\)

=\(\frac{456+344}{100}\)

=\(\frac{800}{100}\)

=8

tk nhé

=800/100

=8

[(x+1)/2]²=26/25-17/25=9/25

=>  (x+1)/2=3/5                =>   x=1/5   hoặc x=-11/5

hoặc (x+1)/2=-3/5

Đề vầy đúng không :

( x+1/2)^2 + 17/25 = 26 / 25

(x+1/2)^2=26/25-17/25

(x+1/2)^2=9/25

=> x+1/2=3/5

=> x = 3/5-1/2=1/10

17 + (x - 7) + 2(2x + 1) = 17

<=> x - 7 + 2(2x + 1) = 17 - 17

<=> x - 7 + 2(2x + 1) = 0

<=> x + 2(2x + 1) = 0 + 7

<=> x + 2(2x + 1) = 7

<=> x + 4x + 1 = 7

<=> 5x + 2 = 7

<=> 5x = 7 - 2

<=> 5x = 5

=> x = 1

\(17+\left(x-7\right)+2\left(2x+1\right)=17\)

\(\Rightarrow x-7+4x+2=0\)

\(\Rightarrow5\left(x-1\right)=0\)

\(\Rightarrow x-1=0\)

\(\Rightarrow x=1\)

\(17+\left(x-7\right)+2\left(2x+1\right)=17\)

\(\Leftrightarrow17+x-7+4x+2=17\)

\(\Leftrightarrow\left(17-7\right)+\left(x+2x\right)+2=17\)

\(\Leftrightarrow10+3x+2=17\)

\(\Leftrightarrow\left(10+2\right)+3x=17\)

\(\Leftrightarrow12+3x=17\)

\(\Leftrightarrow3x=17-12\)

\(\Leftrightarrow3x=5\)

\(\Leftrightarrow x=\frac{5}{3}\)