(-250).(-34) + (-25).68

=(-125).(-68) + 25 . (-68)

= 125 . 68 + (-25) . 68

= 68 . [125 + (-25)]

=68 . 100

= 6800

mình chỉ trả lời câu đầu thôi còn câu đó nếu giải đúng thì đánh đúng giùm nha

1) Tính nhanh:

\(\left(-250\right).\left(-34\right)+\left(-25\right).68\)

\(=\left(-125\right).\left(-68\right)+\left(-25\right).68\)

\(=125.68+\left(-25\right).68\)

\(=68.\left[125+\left(-25\right)\right]\)

\(=68.100\)

\(=6800\)

Giải:

a) \(\dfrac{-5}{6}-x=\dfrac{7}{12}+\dfrac{-1}{3}\)  

     \(\dfrac{-5}{6}-x=\dfrac{1}{4}\)

               \(x=\dfrac{-5}{6}-\dfrac{1}{4}\) 

               \(x=\dfrac{-13}{12}\) 

b) \(2.\left(x-\dfrac{1}{3}\right)=\left(\dfrac{1}{3}\right)^2+\dfrac{5}{9}\) 

    \(2.\left(x-\dfrac{1}{3}\right)=\dfrac{1}{9}+\dfrac{5}{9}\) 

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

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

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

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

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

c) \(\left|2x-\dfrac{3}{4}\right|-\dfrac{3}{8}=\dfrac{1}{8}\) 

           \(\left|2x-\dfrac{3}{4}\right|=\dfrac{1}{8}+\dfrac{3}{8}\) 

            \(\left|2x-\dfrac{3}{4}\right|=\dfrac{1}{2}\) 

\(\Rightarrow\left[{}\begin{matrix}2x-\dfrac{3}{4}=\dfrac{1}{2}\\2x-\dfrac{3}{4}=\dfrac{-1}{2}\end{matrix}\right.\) 

\(\Rightarrow\left[{}\begin{matrix}x=\dfrac{5}{8}\\x=\dfrac{1}{8}\end{matrix}\right.\) 

d) \(\dfrac{2}{3}x+\dfrac{1}{6}x=3\dfrac{5}{8}\) 

\(x.\left(\dfrac{2}{3}+\dfrac{1}{6}\right)=\dfrac{29}{8}\) 

            \(x.\dfrac{5}{6}=\dfrac{29}{8}\) 

                \(x=\dfrac{29}{8}:\dfrac{5}{6}\) 

                \(x=\dfrac{87}{20}\)

[ ( 6x - 39 ) : 7 ] . 4 = 12

=> ( 6x - 39 ) : 7 = 12 : 4

=> ( 6x - 39 ) : 7 = 3

=> 6x - 39 = 3.7

=> 6x - 39 = 21

=> 6x = 21+39

=> 6x = 60

=> x = 10.

( x : 3 - 4 ) . 5 = 15

=> x : 3 - 4 = 15 : 5

=> x : 3 - 4 = 3

=> x : 3 = 7

=> x = 7.3

=> x = 21

x - [ 42 + (-28) ] = -8

=> x - 14 = -8

=> x = -8 + 14

=> x = 6.

[ (6x - 39): 7] . 4=12

(6x-39) : 7=12:4

(6x -39) :7=3

(6x-39) =3 . 7

6x-39 =21

6x =21 +39

6x=20

x=10

(x :3 -4) .5 =15

(x : 3 -4) =15:5

x:3-4 =3

x : 3=3+4

x:3 =7

x=7 . 3

x=21

\(a.\dfrac{12+x}{42}=\dfrac{35}{42}\Leftrightarrow12+x=35\Leftrightarrow x=23\)

\(b.\dfrac{25-x}{40}=\dfrac{3}{8}\Leftrightarrow\dfrac{25-x}{40}=\dfrac{15}{40}\Leftrightarrow25-x=15\Leftrightarrow x=10\)

\(c.\dfrac{13+x}{20}=\dfrac{3}{4}\Leftrightarrow\dfrac{13+x}{20}=\dfrac{15}{20}\Leftrightarrow13+x=15\Leftrightarrow x=2\)

\(d.\dfrac{23-x}{25}=\dfrac{20}{25}\Leftrightarrow23-x=20\Leftrightarrow x=3\)

a.-1,75-(-\(\dfrac{1}{9}\)-2\(\dfrac{1}{8}\))
-1,75-\(\dfrac{1}{9}+\dfrac{17}{8}\)
\(-\dfrac{7}{4}-\dfrac{1}{9}+\dfrac{17}{8}\)
\(\dfrac{-126}{72}-\dfrac{8}{72}+\dfrac{153}{72}\)
=\(\dfrac{19}{72}\)

b.\(\dfrac{-1}{12}-\left(2\dfrac{5}{8}-\dfrac{1}{3}\right)\)
\(\dfrac{-1}{12}-\left(\dfrac{21}{8}-\dfrac{1}{3}\right)\)
\(\dfrac{-1}{12}-\dfrac{21}{8}+\dfrac{1}{3}\)
\(\dfrac{-2}{24}-\dfrac{63}{24}+\dfrac{64}{24}\)
=\(\dfrac{-1}{24}\)

\(\dfrac{-5}{6}-\left(\dfrac{-3}{8}+\dfrac{1}{10}\right)\)
\(\dfrac{-5}{6}-\dfrac{-3}{8}-\dfrac{1}{10}\)
 

a) 12 : x = 3

     X = 12 : 3

     X = 4

b) 21 : x = 7

     X = 21 : 7

     X = 3

c) 30 : x = 3

     X = 30 : 3

     X = 10

d) X : 7 = 4

     X = 4 x 7

     X = 28

e) 20 : x = 5

     X = 20 : 5

     X = 4

f) X x 6 = 42

     X = 42 : 6

     X = 7

Bài 8:

Ta có: \(A=-x^2+2x+4\)

\(=-\left(x^2-2x-4\right)\)

\(=-\left(x^2-2x+1-5\right)\)

\(=-\left(x-1\right)^2+5\le5\forall x\)

Dấu '=' xảy ra khi x=1