\(\frac{5}{12}=\frac{5}{2^2.3}\rightarrow loại\)

\(\frac{3}{11}\rightarrow loại\)

\(\frac{1}{14}=\frac{1}{2.7}\rightarrow loại\)

\(\frac{15}{30}=\frac{1}{2}\rightarrow nhận\)

Vậy chọn D 

\(a,\frac{1}{2}+\frac{3}{4}x=\frac{1}{4}\)

\(\Rightarrow\frac{3}{4}x=\frac{1}{4}-\frac{1}{2}=-\frac{1}{4}\)

\(\Rightarrow x=-\frac{1}{3}\)

Bạn Hồng có một tờ bìa hình chữ nhật có chiều rộng 1/5 m và chiều dài 1/4 m . bạn Hồng muốn cắt từ tấm bìa đó thành một hình chữ nhật có diện tích bằng một nửa diện tích tấm bìa và độ dài một cạnh là 1/4 m để làm hộp đồ chơi.

a) tính diện tích hình chữ nhật bạn Hồng đã cắt

b) bạn Hồng có thể cắt như thế nào ? Vẽ hình minh họa

a)×+1/ 53 + ×+2 /52 + ×+3/ 51+3 = 0

\(\Rightarrow\frac{x+1}{53}+1+\frac{x+2}{52}+1+\frac{x+3}{51}+1+\frac{3\left(x+54\right)}{\left(x+54\right)}=0\)

\(\Rightarrow\frac{x+54}{53}+\frac{x+54}{52}+\frac{x+54}{51}+\frac{x+54}{\frac{1}{3}\left(x+54\right)}=0\)

\(\Rightarrow\left(x+54\right)\left(\frac{1}{53}+\frac{1}{52}+\frac{1}{51}+\frac{1}{\frac{1}{3}\left(x+54\right)}\right)=0\)

\(\Rightarrow x+54=0\).Do \(\frac{1}{53}+\frac{1}{52}+\frac{1}{51}+\frac{1}{\frac{1}{3}\left(x+54\right)}\ne0\)

=>x=-54

b)×-2/ 72 + ×-3/ 71 + ×-4/ 70 -3 = 0

\(\Rightarrow\frac{x-2}{72}-1+\frac{x-3}{71}-1+\frac{x-4}{70}-1-\frac{3\left(x-74\right)}{x-74}=0\)

\(\Rightarrow\frac{x-74}{72}+\frac{x-74}{71}+\frac{x-74}{70}-\frac{x-74}{\frac{1}{3}\left(x-74\right)}=0\)

\(\Rightarrow\left(x-74\right)\left(\frac{1}{72}+\frac{1}{71}+\frac{1}{70}-\frac{1}{\frac{1}{3}\left(x-74\right)}\right)=0\)

\(\Rightarrow x-74=0\).Do \(\frac{1}{72}+\frac{1}{71}+\frac{1}{70}-\frac{1}{\frac{1}{3}\left(x-74\right)}\ne0\)

=>x=74

c)×+5/ 81 + ×+4/ 41 + ×-7/ 31 + 6 = 0

\(\Rightarrow\frac{x+5}{81}+1+\frac{x+4}{41}+2+\frac{x-7}{31}+3+\frac{6\left(x+86\right)}{x+86}=0\)

\(\Rightarrow\frac{x+86}{81}+\frac{x+86}{41}+\frac{x+86}{31}+\frac{x+86}{\frac{1}{6}\left(x+86\right)}=0\)

\(\Rightarrow\left(x+86\right)\left(\frac{1}{81}+\frac{1}{41}+\frac{1}{31}+\frac{1}{\frac{1}{6}\left(x+86\right)}\right)=0\)

\(\Rightarrow x+86=0\).Do \(\frac{1}{81}+\frac{1}{41}+\frac{1}{31}+\frac{1}{\frac{1}{6}\left(x+86\right)}\ne0\)

=>x=-86

d)tương tự nhé

a) Ta có :

\(0,\left(27\right)+0,\left(72\right)==\dfrac{27}{99}+\dfrac{72}{99}=\dfrac{99}{99}=1\)

\(\Rightarrow0,\left(27\right)+0,\left(72\right)=1\rightarrowđpcm\)

b) Ta có :

\(0,\left(22\right).\dfrac{9}{2}=\dfrac{2}{9}.\dfrac{9}{2}=\dfrac{18}{18}=1\)

\(\Rightarrow0,22.\dfrac{9}{2}=1\rightarrowđpcm\)

c) Ta có :

\(\left[0,\left(11\right).9\right]^{2003}=\left[\dfrac{1}{9}.9\right]^{2003}=\left[\dfrac{9}{9}\right]^{2003}=1^{2003}=1\)

\(\Rightarrow\left[0,\left(11\right).9\right]^{2003}=1\rightarrowđpcm\)

a) \(0,\left(27\right)+0,\left(72\right)=0,\left(99\right)=1\)

b) \(0,\left(22\right)\cdot\dfrac{9}{2}=\dfrac{2}{9}\cdot\dfrac{9}{2}=1\)

c) \(\left[0,\left(11\right)\cdot9\right]^{2003}=\left(\dfrac{1}{9}\cdot9\right)^{2003}=1^{2003}=1\)

1.

a) \(\frac{-7}{9}.2\frac{3}{4}=\frac{-7}{9}.\frac{11}{4}=\frac{-77}{36}\)

b) \(\frac{2}{3}+\frac{1}{3}.\frac{-2}{5}=\frac{2}{3}+\frac{-2}{15}=\frac{8}{15}\)

c) \(\frac{3}{4}.15\frac{1}{3}-\frac{3}{4}.43\frac{1}{3}=\frac{3}{4}.\frac{46}{3}-\frac{3}{4}.\frac{130}{3}=\frac{23}{2}-\frac{65}{2}=-21\)

d) \(\left(-49,1\right).\frac{13}{27}-58,9.\frac{13}{27}=\frac{13}{27}.\left(-49,1-58,9\right)=\frac{13}{27}.\left(-108\right)=-52\)

e) \(0,375:\left(-4,5\right)=\frac{-1}{12}\)

f) \(3\frac{1}{7}:\left(-1\frac{3}{7}\right)=\frac{22}{7}:\frac{-10}{7}=\frac{-11}{5}\)

g) \(9\frac{1}{3}:4\frac{2}{3}-2=\frac{28}{3}:\frac{14}{3}-2=2-2=0\)

h) \(\left(7\frac{3}{4}:0,3125+4,5.2\frac{2}{45}\right):\left(-8,5\right)=\left(\frac{31}{4}:\frac{5}{16}+\frac{9}{2}.\frac{92}{45}\right):\frac{-17}{2}=\left(\frac{124}{5}+\frac{46}{5}\right):\frac{-17}{2}=34:\frac{-17}{2}=-4\)

Bài 1 : Tính:

a)

\(\frac{-7}{9}.2\frac{3}{4}=\frac{-7}{9}.\frac{11}{4}=\frac{-77}{36}\)

b) 

\(\frac{2}{3}+\frac{1}{3}.\frac{-2}{5}=\frac{2}{3}+\frac{-2}{15}=\frac{10}{15}+\frac{-2}{15}=\frac{8}{15}\)

c)

\(\frac{3}{4}.15\frac{1}{3}-\frac{3}{4}.43\frac{1}{3}=\frac{3}{4}.\frac{46}{3}-\frac{3}{4}.\frac{130}{3}\)\(=\frac{23}{2}-\frac{65}{2}=\frac{-42}{2}=-21\)

....

Tự lm tiếp dạng như v

Bài 2 : 

\(A=\frac{-6}{11}.\frac{7}{10}.\frac{11}{-6}.-20=\left(\frac{-6}{11}.\frac{11}{-6}\right).\left(\frac{7}{10}.-20\right)\)\(=1.\left(-14\right)=-14\)

.....

Bài 3 : 

\(\frac{3}{7}.x-\frac{2}{5}.x=\frac{-17}{35}\)

\(\Leftrightarrow\frac{3}{7}-\frac{2}{5}.x=\frac{-17}{35}\)

\(\Leftrightarrow\frac{1}{35}x=\frac{-17}{35}\)

\(\Leftrightarrow x=\frac{-17}{35}:\frac{1}{35}\)

\(\Leftrightarrow x=\frac{-17}{35}.35=-17\)

a) Ta có:

C = 5/18 + 8/19 - 7/21 + (-10/36 + 11/19 + 1/3) - 5/8

C = 5/18 + 8/19 - 1/3 - 5/18 + 11/19 + 1/3 - 5/8

C = (5/18 - 5/18) + (8/19 + 11/19) - (1/3  - 1/3) - 5/8

C = 1 - 5/8

c = 3/8

b) F = 15/14 - (17/23 - 80/87 + 5/4) + (17/23 - 15/14 + 1/4)

F = 15/14 - 17/23 + 80/87 - 5/4 + 17/23 - 15/14 + 1/4

F = (15/14 - 15/14) - (17/23 - 17/23) + 80/87 - (5/4 - 1/4)

F = 80/87 - 1

F = -7/87

c) G = 1/25 - 4/27 + (-23/27 + -1/25 - 5/43) + 5/43 - 4/7

G = 1/25 - 4/27 - 23/27 - 1/25 - 5/43 + 5/43 - 4/7

G = (1/25 - 1/25) - (4/27 + 23/27) - (5/43 - 5/43) - 4/7

G = -1 - 4/7 = -11/7

a) \(\frac{2}{3}-4.\left(\frac{1}{2}+\frac{3}{4}\right)\)

= \(\frac{2}{3}-4.\frac{5}{4}\)

= \(\frac{2}{3}-5\)

= \(-\frac{13}{3}\).

b) Câu này là 117 hay \(\frac{11}{7}\) vậy bạn?

Chúc bạn học tốt!

OK bạn.