Bài 2: Cộng, trừ số hữu tỉ

\(\left(-1\right)+1=0\)

It's Very easy!!!

(-1) + 1 = 0

[Bạn rảnh thiệt á -_-]

(-1) + 1= 1 + (-1) = 0

a: \(\Leftrightarrow\dfrac{23}{5}\cdot\dfrac{50}{23}< =x< =\dfrac{-13}{5}:\dfrac{7}{5}\)

=>10<=x<=-13/7

hay \(x\in\varnothing\)

b: \(\Leftrightarrow-\dfrac{13}{3}\cdot\dfrac{1}{3}< =x< =\dfrac{-2}{3}\cdot\dfrac{-11}{12}\)

=>-13/9<=x<=22/36

hay \(x\in\left\{-1;0\right\}\)

\(\dfrac{1}{100.99}-\dfrac{1}{99.98}-\dfrac{1}{98.97}-...-\dfrac{1}{3.2}-\dfrac{1}{2.1}\)

\(=\dfrac{1}{100}-\dfrac{1}{99}+\dfrac{1}{99}-\dfrac{1}{98}+\dfrac{1}{98}-\dfrac{1}{97}+...+\dfrac{1}{3}-\dfrac{1}{2}+\dfrac{1}{2}-1\)

\(=\dfrac{1}{100}-1\)

\(=-\dfrac{99}{100}\)

Bài này chỉ cần tìm ra số của x là làm được

\(\dfrac{x+1}{9}+\dfrac{x+1}{8}+\dfrac{x+1}{7}=\dfrac{x+1}{6}\)

=>x+1=0

=>x=-1

Bài 2: 

a: Để x>0 thì a-3>0

=>a>3

b: Để x<0 thì a-3<0

=>a<3

c: Để x=0 thì a-3=0

=>a=3

Bài 1: 

\(=\left(\dfrac{1}{3}+\dfrac{3}{5}+\dfrac{1}{15}\right)+\left(\dfrac{-3}{4}-\dfrac{2}{9}-\dfrac{1}{36}\right)+\dfrac{1}{64}\)

\(=\dfrac{5+9+1}{15}+\dfrac{-27-8-1}{36}+\dfrac{1}{64}\)

\(=\dfrac{1}{64}\)

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

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

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

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

\(=-\dfrac{5}{2}\)

dễ òm

\(x-\dfrac{1}{2}=-\dfrac{2}{3}\\ x=-\dfrac{2}{3}+\dfrac{1}{2}\\ x=\dfrac{-1}{6}\) đây mà là toán lớp 7

\(\dfrac{2}{7}-x=-\dfrac{3}{4}\\ x=\dfrac{2}{7}+\dfrac{3}{4}\\ x=\dfrac{29}{28}\)

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

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

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

Vậy ..................

\(\dfrac{2}{7}-x=-\dfrac{3}{4}\)

\(x=\dfrac{2}{7}+\dfrac{3}{4}\)

\(x=\dfrac{29}{28}\)

Vậy ..................

\(a.\) \(A=\left(15x+2y\right)-\left[\left(2x+3\right)-\left(5x+y\right)\right]\)

\(A=15x+2y-2x-3+5x+y\)

\(A=\left(15x-2x+5x\right)+\left(2y+y\right)-3\)

\(A=18x+3y-3\)

\(A=3\left(6x+y-1\right)\)

\(b.\) \(B=-\left(12x+3y\right)+\left(5x-2y\right)-\left[13x+\left(2y-5\right)\right]\)

\(B=-12x-3y+5x-2y-13x-2y+5\)

\(B=-\left(12x-5x+13x\right)-\left(3y+2y+2y\right)+5 \)

\(B=-20x-7y+5\)

A = (0,87 + 0,64) . (1,25 . 7 + \(\dfrac{4}{5}\) . 1,25) + 31,64

A = 1,51 . [1,25 . (7 + \(\dfrac{4}{5}\))] + 31,64

A = 1,51 . (1,25 . 7,8) + 31,64

A = 1,51 . 9,75 + 31,64

A = 14,7225 + 31,64

A = 46,3625

A = \(\dfrac{3709}{80}\)

A= (0,87+0,64).(1,25.7+\(\dfrac{4}{5}\).1,25)+31,64

= 1,51.(1,25.(7+\(\dfrac{4}{5}\)))+31.64

= 1,51.(1,25.\(\dfrac{39}{5}\))+31.64

= 1,51.\(\dfrac{39}{4}\)+31.64

= \(\dfrac{5889}{400}\)+31,64

= 46,3625