Gọi vận tốc ca nô là x ( x > 0 ) 

Theo bài ra ta có pt \(\dfrac{72}{x+3}+\dfrac{54}{x-3}=6\Rightarrow x=21\left(tm\right)\)

c: Ta có: \(\dfrac{1}{x^2+x+1}-\dfrac{1}{x-x^2}+\dfrac{2x}{1-x^3}\)

\(=\dfrac{1}{x^2+x+1}+\dfrac{1}{x\left(x-1\right)}-\dfrac{2x}{\left(x-1\right)\left(x^2+x+1\right)}\)

\(=\dfrac{x^2-x+x^2+x+1-2x^2}{x\left(x-1\right)\left(x^2+x+1\right)}\)

\(=\dfrac{1}{x\left(x-1\right)\left(x^2+x+1\right)}\)

Đây bạn nhé!

a) Do \(\left(3x-\dfrac{1}{2}\right)^2\ge0\forall x\)

 \(\Rightarrow A=\left(3x-\dfrac{1}{2}\right)^2-4\ge-4\)

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

b) Do \(\left(2x+1\right)^4\ge0\forall x,\left(y-\dfrac{1}{2}\right)^6\ge0\forall y\)

\(\Rightarrow B=\left(2x+1\right)^4+3\left(y-\dfrac{1}{2}\right)^6\ge0\)

\(minB=0\Leftrightarrow\)\(\left\{{}\begin{matrix}x=-\dfrac{1}{2}\\y=\dfrac{1}{2}\end{matrix}\right.\)

a: \(A=\left(3x-\dfrac{1}{2}\right)^2-4\ge-4\forall x\)

Dấu '=' xảy ra khi \(x=\dfrac{1}{6}\)

b: \(B=\left(2x+1\right)^4+3\left(y-\dfrac{1}{2}\right)^6\ge0\forall x,y\)

Dấu '=' xảy ra khi \(\left(x,y\right)=\left(-\dfrac{1}{2};\dfrac{1}{2}\right)\)

\(-\dfrac{4}{5}+2x=\dfrac{1}{3}-\dfrac{2}{3}x\\ \Leftrightarrow-\dfrac{4}{5}+2x-\dfrac{1}{3}+\dfrac{2}{3}x=0\\ \Leftrightarrow-\dfrac{17}{15}+\dfrac{8}{3}x=0\\ \Leftrightarrow\dfrac{8}{3}x=\dfrac{17}{15}\\ \Leftrightarrow x=\dfrac{17}{40}\)

=>8/3x=1/3+4/5=5/15+12/15=17/15

=>x=17/15:8/3=17/15x3/8=51/120=17/40

c) \(\dfrac{2x-1}{3}=\dfrac{27}{2x-1}\)=>(2x-1)²=27.3=81

=>2x-1=9 hay 2x-1=-9

=>x=5 hay x=-4

ko có ảnh bn ơi

Đề bài là gì zạ? 

is your pen blue or green

is your computer grey or black

i love football because it's interesting

i love guitar because very easy

you so funny

you so interesting

Or: Do you like to eat pizza or hamburgers?

Do you like red or yellow?

Because: I like elephants because they are friendly.

I don't go to school today because I'm sick.

So: I like to eat apples so much.

Thank you so much!

BTVN Nâng cao thì tự làm hoặc là  ghi không biết vào đó

Chứ t chịu

Bài 4: 

a) Ta có: \(\dfrac{x}{13}=\dfrac{35}{91}\)

\(\Leftrightarrow\dfrac{x}{13}=\dfrac{5}{13}\)

hay x=5

Vậy: x=5

b) Ta có: \(\dfrac{9+x}{13-x}=\dfrac{5}{6}\)

\(\Leftrightarrow6\left(x+9\right)=5\left(13-x\right)\)

\(\Leftrightarrow6x+54=65-5x\)

\(\Leftrightarrow6x+5x=65-54\)

\(\Leftrightarrow11x=11\)

hay x=1

Vậy: S={1}

c) Ta có: \(x+x:5\cdot7.5+x:2\cdot9=315\)

\(\Leftrightarrow x+\dfrac{1}{5}x\cdot\dfrac{15}{2}+\dfrac{1}{2}x\cdot9=315\)

\(\Leftrightarrow7x=315\)

hay x=45

Vậy: x=45