Bài 1: Tìm x.

a. 7x - 5 = 16

⇒ 7x = 16 + 5

⇒ 7x = 21

=> x = 21 : 7

=> x = 3

Vậy : x = 3

b. 156 - 2 = 82

c. 10x + 65 = 125

=> 10x = 125 - 65

=> 10x = 60

=> x = 60 : 10

=> x = 6

Vậy : x = 6

e. 15 + 5x = 40

=> 5x = 40 -15

=> 5x = 25

=> x = 25 : 5

=> x = 5

Vậy : x = 5

F=(9.75.21\(\dfrac{3}{7}\)+\(\dfrac{39}{4}\).18\(\dfrac{4}{7}\)).\(\dfrac{15}{78}\)

=(\(\dfrac{39}{4}\).21\(\dfrac{3}{7}\)+\(\dfrac{39}{4}\).18\(\dfrac{4}{7}\)).\(\dfrac{15}{78}\)

=[\(\dfrac{39}{4}\).(21\(\dfrac{3}{7}\)+18\(\dfrac{4}{7}\))].\(\dfrac{15}{78}\)

=[\(\dfrac{39}{4}\).(21+18)+(\(\dfrac{3}{7}\)+\(\dfrac{4}{7}\))].\(\dfrac{15}{78}\)

=[\(\dfrac{39}{4}\).(39+1)].\(\dfrac{15}{78}\)

=(\(\dfrac{39}{4}\).40).\(\dfrac{15}{78}\)

=390.\(\dfrac{15}{78}\)=75

\(B=71\dfrac{38}{45}-\left(43\dfrac{8}{45}-1\dfrac{17}{57}\right)\)

\(B=71\dfrac{38}{45}-43\dfrac{8}{45}-1\dfrac{17}{57}\)

\(B=28\dfrac{2}{3}-1\dfrac{17}{57}=27\dfrac{11}{57}\)

\(D=\left(19\dfrac{5}{8}:\dfrac{7}{12}-13\dfrac{1}{4}:\dfrac{7}{12}\right).\dfrac{4}{5}\)

\(D=\dfrac{12}{7}.\left(19\dfrac{5}{8}-13\dfrac{1}{4}\right).\dfrac{4}{5}\)

\(D=\dfrac{12}{7}.\dfrac{51}{8}.\dfrac{4}{5}=\dfrac{306}{35}\)

Câu còn lại làm tương tự!

Chúc bạn học tốt!!!

a)

\(\overline{5\circledast8}⋮3khi\left(5+\circledast+8\right)⋮3\Rightarrow\left(13+\circledast\right)⋮3\)

\(\Rightarrow\circledast\) = 2 hoặc \(\circledast\) = 5 hoặc \(\circledast\) = 8.

Vậy chữ số thay cho \(\circledast\) là 2 hoặc 5 hoặc 8.

b)

\(\overline{6\circledast3}⋮9khi\left(6+3+\circledast\right)⋮9\Rightarrow\left(9+\circledast\right)⋮9\)

\(\Rightarrow\circledast\) = 0 hoặc \(\circledast\) = 9.

Vậy chữ số thay \(\circledast\) là 0 hoặc 9

c)

\(\overline{43\circledast}⋮3khi\left(4+3+\circledast\right)⋮3\Rightarrow\circledast=2\text{hoặc}\circledast=5\text{hoặc}\circledast=8\left(1\right)\)

\(\overline{43\circledast}⋮5khi\circledast=0\text{hoặc}\circledast5\)

Vì \(\circledast\) phải thỏa mãn (1) và ( 2) nên \(\circledast\) = 5.

d)

Vì \(\overline{\circledast81\circledast}⋮5\) nên dấu \(\circledast\) ở hàng đơn vị phải bằng 0 hoặc 5

Mà \(\overline{\circledast81\circledast}⋮2\) nên dấu \(\circledast\) ở hàng đơn vị phải bằng 0 ( vì 5 là số lẻ ) . Thay vào ta được số : \(\overline{\circledast810}\)

Để \(\overline{\circledast810}⋮9\) thì \(\left(\circledast+8+1+0\right)⋮9=\left(\circledast+9\right)\Rightarrow\circledast=0\text{hoặc}\circledast=9\)

Mà \(\circledast\) lại là số ở hàng nghìn (là số đầu tiên) nên \(\circledast\) ≠ 0. Do đó \(\circledast\) = 9

Vậy ta được số 9810

a)5

b)9

c)5

d)90

a) 0

b) 0

c) 1

d) 0

a, Với \(-841< -84\circledast=>\circledast=0\)

b, Với \(-5\circledast8>-518=>\circledast=0\)

c, Với \(-\circledast8>-25=>\circledast=1\)

d, Với\(-99\circledast>-991=>\circledast=0\)

B)

a) \(\dfrac{11}{21}+\dfrac{-4}{7}=\dfrac{11}{21}+\dfrac{-12}{21}=\dfrac{-1}{21}\)

b) \(\dfrac{5}{15}+\dfrac{14}{25}-\dfrac{12}{9}+\dfrac{2}{7}+\dfrac{11}{25}=\dfrac{1}{3}+\dfrac{14}{25}-\dfrac{4}{3}+\dfrac{2}{7}+\dfrac{11}{25}\)

\(=\left(\dfrac{1}{3}-\dfrac{4}{3}\right)+\left(\dfrac{14}{25}+\dfrac{11}{25}\right)+\dfrac{2}{7}=-1+1+\dfrac{2}{7}=\dfrac{2}{7}\)

c) \(\dfrac{2}{3}+\dfrac{5}{7}-\dfrac{3}{14}=\dfrac{28}{42}+\dfrac{30}{42}-\dfrac{9}{42}=\dfrac{49}{42}=\dfrac{7}{6}\)

d) \(\dfrac{2}{5}-\dfrac{3}{7}+\dfrac{9}{45}=\dfrac{2}{5}-\dfrac{3}{7}+\dfrac{1}{5}=\dfrac{14}{35}-\dfrac{15}{35}+\dfrac{7}{35}=\dfrac{6}{35}\)

e) \(\dfrac{21}{47}+\dfrac{9}{45}+\dfrac{26}{47}+\dfrac{45}{5}=\dfrac{21}{47}+\dfrac{1}{5}+\dfrac{26}{47}+\dfrac{45}{5}=\left(\dfrac{21}{47}+\dfrac{26}{47}\right)+\left(\dfrac{1}{5}+\dfrac{45}{5}\right)\)

\(=1+\dfrac{46}{5}=\dfrac{51}{5}\)

f) \(\dfrac{15}{12}-\dfrac{18}{13}+\dfrac{5}{13}-\dfrac{3}{12}=\left(\dfrac{15}{12}-\dfrac{3}{12}\right)+\left(-\dfrac{18}{13}+\dfrac{5}{13}\right)=1+\left(-1\right)=0\)

g) \(\dfrac{-8}{18}-\dfrac{15}{27}=\dfrac{-4}{9}-\dfrac{5}{9}=\dfrac{-9}{9}=-1\)

h)\(\dfrac{3}{7}+\dfrac{-5}{2}-\dfrac{3}{5}=\dfrac{30}{70}+\dfrac{-175}{70}-\dfrac{42}{70}=\dfrac{-187}{70}\)

i) \(\left(\dfrac{11}{12}:\dfrac{33}{16}\right).\dfrac{3}{5}=\dfrac{11}{12}.\dfrac{16}{33}.\dfrac{3}{5}=\dfrac{11.16.3}{12.33.5}=\dfrac{4}{15}\)

a) 7/15-(2/15-12/18)

=7/15-2/15+2/3

=5/15+2/3

=1/3+2/3

=1

b)(7/41- 4/9) - (3/19 + 7/41) + (4/9 - 16/19)

=7/41 - 4/9 - 3/19 - 7/41 + 4/9 + 16/19

=(7/41 - 7/41) - (4/9 - 4/9) - (3/19 + 16/19)

= -1

b) 7/41 - 4/9 - 3/19 - 7/41 + 4/9 + 16/19

= (7/41 - 7/41 ) - (4/9 - 4/9 ) + ( 3/19 + 16/19 )

= 0 - 0 + 1

= 1

a)1230

b)6850

c)5745

d)4960