giúp i mn ưiiiii

Haizzz Dễ Thôi Chịu:)

a)\(\dfrac{2}{5}+\dfrac{3}{4}:x=\dfrac{-1}{2}\)

=>\(\dfrac{3}{4}:x=\dfrac{-1}{2}-\dfrac{2}{5}\)

=>\(\dfrac{3}{4}:x=\dfrac{-9}{10}\)

=>\(x=\dfrac{3}{4}:\dfrac{-9}{10}\)

=>\(x=\dfrac{3}{4}.\dfrac{-10}{9}\)

=>\(x=\dfrac{-5}{6}\)

b)\(\dfrac{5}{7}-\dfrac{2}{3}.x=\dfrac{4}{5}\)

=>\(\dfrac{2}{3}.x=\dfrac{5}{7}-\dfrac{4}{5}\)

=>\(\dfrac{2}{3}.x=\dfrac{-3}{35}\)

=>\(x=\dfrac{-3}{35}:\dfrac{2}{3}\)

=>\(x=\dfrac{-3}{35}.\dfrac{3}{2}\)

=>\(x=\dfrac{-9}{70}\)

c)\(\dfrac{1}{2}x+\dfrac{3}{5}x=\dfrac{-2}{3}\)

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

=>\(\dfrac{11}{10}x=\dfrac{-2}{3}\)

=>\(x=\dfrac{-2}{3}:\dfrac{11}{10}\)

=>\(x=\dfrac{-2}{3}.\dfrac{10}{11}\)

=>\(x=\dfrac{-20}{33}\)

d)\(\dfrac{4}{7}x-x=\dfrac{-9}{14}\)

=>\(x\left(\dfrac{4}{7}-1\right)=\dfrac{-9}{14}\)

=>\(\dfrac{-3}{7}x=\dfrac{-9}{14}\)

=>\(x=\dfrac{-9}{14}:\dfrac{-3}{7}\)

=>\(x=\dfrac{-9}{14}.\dfrac{-7}{3}\)
=>\(x=\dfrac{3}{2}\)

1/4 x ( x-1) = 1/2 - 3/4 

1/4 x ( x-1) = -1/4

x - 1 = -1/4 : 1/4

x-1 = -1

x = -1+1

x = 0

Câu 1: C

Câu 2: B

Câu 3:C

Câu 4: A

Câu 5:B

Câu 6: C

Câu 7: B

bài 2: - DT:mặt trời, bụi tre, buổi chiều, bò, sông, nước, bóng, anh, bạn

- ĐT:rúc, về, ra, uống, thấy, chào, gặp

- TT: mát

\(\Leftrightarrow\dfrac{1}{2}\left(\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4}+...+\dfrac{1}{x}-\dfrac{1}{x+1}\right)=1+\dfrac{1989}{1991}\)

\(\Leftrightarrow\dfrac{1}{2}-\dfrac{1}{x+1}=\dfrac{7960}{1991}\)

=>1/x+1=-13929/3982

=>x+1=-3982/13929

hay x=-17911/13929

\(x\times\)2,5 + 2,5 \(\times\) 0,6  = 1,8 : 0,2

\(x\)\(\times\)2,5 + 1,5 = 9

\(x\) \(\times\) 2,5 = 9 - 1,5

\(x\times\)2,5 = 7,5

\(x\)         = 7,5 : 2,5

\(x\)         =  3

\(x\times2,5+2,5\times0,6=1,8:0,2\\ 2,5\times\left(x+0,6\right)=9\\ x+0,6=9:2,5\\ x+0,6=3,6\\ x=3,6-0,6\\ x=3\)

dạ con cảm ơn cô ạ

Ta có: `x/3=y/2 -> x/9=y/6`

`y/3=z/4 -> y/6=z/8`

Từ `2` điều trên `-> x/9=y/6=z/8`

Áp dụng tính chất dãy tỉ số bằng nhau ta có:

`x/9=y/6=z/8=(x+y+z)/(9+6+8)=46/23=2`

`-> x/9=y/6=z/8=2`

`-> x=2*9=18, y=2*6=12, z=2*8=16.`