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

\(\dfrac{3}{4}\times x=\dfrac{4}{5}\times\dfrac{1}{2}\)

\(\dfrac{3}{4}\times x=\dfrac{2}{5}\)

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

\(x=\dfrac{8}{15}\)

b) \(\left(14-x:6\right)\times7=84\)

\(14-x:6=84:7\)

\(14-x:6=12\)

\(x:6=14-12\)

\(x:6=2\)

\(x=6\times2=12\)

c) \(x\times1,5:2,4=0,3\)

\(x\times1,5=0,3\times2,4\)

\(x\times1,5=0,72\)

\(x=0,72:1,5\)

\(x=0,48\)

d) Thiếu x

b.

ĐKXĐ: \(x\ge-1\)

\(\sqrt{\left(x+1\right)\left(x+35\right)}-14\sqrt{x+35}+84-6\sqrt{x+1}=0\)

\(\Leftrightarrow\sqrt{x+1}\left(\sqrt{x+35}-14\right)-6\left(\sqrt{x+35}-14\right)=0\)

\(\Leftrightarrow\left(\sqrt{x+1}-6\right)\left(\sqrt{x+35}-14\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}\sqrt{x+1}=6\\\sqrt{x+35}=14\end{matrix}\right.\)

\(\Leftrightarrow...\)

a. ĐKXĐ: \(-1\le x\le1\)

Đặt \(\left\{{}\begin{matrix}\sqrt{x+1}=a\ge0\\\sqrt{1-x}=b\ge0\end{matrix}\right.\)

\(\Rightarrow a+2a^2=-b^2+b+3ab\)

\(\Leftrightarrow\left(2a^2-3ab+b^2\right)+a-b=0\)

\(\Leftrightarrow\left(a-b\right)\left(2a-b\right)+a-b=0\)

\(\Leftrightarrow\left(a-b\right)\left(2a-b+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}a=b\\2a+1=b\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}\sqrt{x+1}=\sqrt{1-x}\\2\sqrt{x+1}+1=\sqrt{1-x}\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\4x+5+4\sqrt{x+1}=1-x\left(1\right)\end{matrix}\right.\)

(1) \(\Leftrightarrow4\sqrt{x+1}=-4-5x\) \(\left(x\le-\dfrac{4}{5}\right)\)

\(\Leftrightarrow16\left(x+1\right)=25x^2+40x+16\)

\(\Leftrightarrow25x^2+24x=0\Rightarrow\left[{}\begin{matrix}x=0\left(loại\right)\\x=-\dfrac{24}{25}\end{matrix}\right.\)

ĐK: \(x\ge-1\)

pt <=> \(\left(14\sqrt{x+35}-84\right)+\left(6\sqrt{x+1}-\sqrt{x^2+36x+35}\right)=0\)

<=> \(14\left(\sqrt{x+35}-6\right)+\sqrt{x+1}\left(6-\sqrt{x+35}\right)=0\)

<=> \(\left(\sqrt{x+35}-6\right)\left(11-\sqrt{x+1}\right)=0\)

<=> \(\orbr{\begin{cases}\sqrt{x+35}-6=0\\11-\sqrt{x+1}=0\end{cases}}\)Em làm tiếp nhé!

( 14 - x : 6 ) x 7 = 84

14 - x : 6 = 84 : 7

14 - x : 6 = 12

x : 6 = 14 - 12

x : 6 = 2

     x = 2 x 6

     x = 12

14 - x:6 = 84:7

14 - x:6=12

x:6=14-12

x:6=2

x=2x6

x=12

14-x:6= 84:7 =12

x:6=12+14=26

x= ...??

( 14 - x : 6 ) x 7 = 84

14 - x : 6 = 84 : 7

14 - x : 6 = 12

x : 6 = 14 - 12

x : 6 = 2

     x = 2 x 6

     x = 12

( 14 - x : 6 ) x 7 = 84

14 - x : 6 = 84 : 7

14 - x : 6 = 12

x : 6 = 14 - 12

x : 6 = 2

     x = 2 x 6

     x = 12

.

:)