\(x+\left(x+5\right)+\left(x+10\right)+\left(x+15\right)+\left(x+20\right)+...+\left(x+40\right)=288\\ \left(x+x+x+x+x+...+x\right)+\left(5+10+15+20+...+40\right)=288\\ 9\times x+180=288\\ 9\times x=108\\ x=108:9=12\)

các số thập phân: 0,168; 0,186; 0,861; 0,816; 0,681; 0,618

tổng : 0,168+ 0,186+ 0,861+ 0,816+ 0,681+ 0,618 = 3,33

thank you

Bài `1:`

`A=(a+2b+c)-(a-3b)+(5b+2c)`

`A=a+2b+c-a+3b+5b+2c`

`A=(a-a)+(2b+3b+5b)+(c+2c)`

`A=10b+3c`

~~~~~~~~~~~~~~~~~~~~~~~~

`B=3a-b-(2a-5b+c)+(c-4b)`

`B=3a-b-2a+5b-c+c-4b`

`B=(3a-2a)+(-b+5b-4b)+(-c+c)`

`B=a`

~~~~~~~~~~~~~~~~~~~~~~~

`C=a-2b+(3a+b-4c)-(5a-b+3c)`

`C=a-2b+3a+b-4c-5a+b-3c`

`C=(a+3a-5a)+(-2b+b+b)+(-4c-3c)`

`C=-a-7c`

~~~~~~~~~~~~~~~~~~~~~~~

`D=(2b-3a+c)-(a-4b+2c)-(6b-3a-c)`

`D=2b-3a+c-a+4b-2c-6b+3a+c`

`D=(-3a-a+3a)+(2b+4b-6b)+(c-2c+c)`

`D=-a`

_______________________________________________________

Bài `2:`

`a)332-(681+232)+(431-50)`

`=332-681-232+431-50`

`=(332-232)-(681-431)-50`

`=100-250-50`

`=100-(250+50)=100-300=-200`

~~~~~~~~~~~~~~~~~~~~~~~~~~~

`b)-137.(136-417)-136.(417-137)`

`=-137.136-137.(-417)-136.417-136.(-137)`

`=[-137.136-136.(-137)]+[-137.(-417)-136.417]`

`=-137.(136-136)-417.(-137+136)`

`=-137.0-417.(-1)`

`=0+417=417`

A = (a + 2b + c) - (a - 3b) + (5b + 2c)

   = a + 2b + c - a + 3b + 5 b + 2c

  = a - a + 2b + 3b + 5b + c + 2c

=  10b + 3c

B = 3a - b - (2a - 5b + c) + (c - 4b)

    = 3a - b - 2a + 5b - c + c - 4b

    = 3a - 2a - b + 5b - 4b - c + c

    =  a 

C = a - 2b + (3a +b - 4c) - (5a - b + 3c)

   = a - 2b + 3a + b - 4c - 5a + b - 3c

  = a + 3a - 5a - 2b + b + b - 4c - 3c

 =  - a - 7c

D= ( 2b - 3a + c) - (a - 4b + 2c) - ( 6b - 3a - c) 

  =  2b - 3a + c - a + 4b - 2c -  6b + 3a + c

  =  - 3a - a + 3a + 2b + 4b - 6b + c - 2c + c

 =  - a 

     Số các số lẻ có 2 chữ số:

     (99 - 11) : 2 + 1 = 45 (số)

    Tổng các số lẻ có 2 chữ số:

   (99 + 11) . 45 : 2 = 2475

     Số các số chẵn có 2 chữ số:

  (98 - 10) : 2 + 1 = 45 (số)

Tổng các số chẵn có 2 chữ số:

    (98 + 10) . 45 : 2 = 2430

Hiệu của chúng là: 2475 - 2430 = 45

                       Đáp số:.............

\(\dfrac{2020x2018+9}{2019x2020-2011}=\dfrac{2020x\left(2019-1\right)+9}{2020x2019-2011}=\dfrac{2020x2019-2020+9}{2020x2019-2011}=\dfrac{2020x2019-2011}{2020x2019-2011}=1\)\(\left(1+\dfrac{1}{2}\right)x\left(1+\dfrac{1}{3}\right)x\left(1+\dfrac{1}{4}\right)x...x\left(1+\dfrac{1}{2020}\right)\\ =\dfrac{3}{2}x\dfrac{4}{3}x\dfrac{5}{4}x...x\dfrac{2021}{2020}=\dfrac{2021}{2}\)

a) \(\dfrac{15}{13}>\dfrac{13}{13}=>\dfrac{15}{13}>1\\ \dfrac{35}{37}< \dfrac{37}{37}=>\dfrac{35}{37}< 1\\ =>\dfrac{15}{13}>\dfrac{35}{37}\)

b)\(\dfrac{19}{20}< \dfrac{20}{20}=>\dfrac{19}{20}< 1\\ \dfrac{39}{38}>\dfrac{38}{38}=>\dfrac{39}{38}>1\\ =>\dfrac{19}{20}< \dfrac{39}{38}\)