a: ĐKXĐ: \(\left(x+2\right)\left(x+3\right)>=0\)

=>\(\left[{}\begin{matrix}x>=-2\\x< =-3\end{matrix}\right.\)

\(y=\sqrt{\left(x+2\right)\left(x+3\right)}=\sqrt{x^2+5x+6}\)

=>\(y'=\dfrac{\left(x^2+5x+6\right)'}{2\sqrt{x^2+5x+6}}=\dfrac{2x+5}{2\sqrt{x^2+5x+6}}\)

y'>0

=>\(\dfrac{2x+5}{2\sqrt{x^2+5x+6}}>0\)

=>2x+5>0

=>\(x>-\dfrac{5}{2}\)

Kết hợp ĐKXĐ, ta được: x>=-2

Đặt y'<0

=>2x+5<0

=>2x<-5

=>\(x< -\dfrac{5}{2}\)

Kết hợp ĐKXĐ, ta được: x<=-3

Vậy: Hàm số đồng biến trên \([-2;+\infty)\) và nghịch biến trên \((-\infty;-3]\)

b: ĐKXĐ: \(\dfrac{2x+1}{x-3}>=0\)

=>\(\left[{}\begin{matrix}x>3\\x< =-\dfrac{1}{2}\end{matrix}\right.\)

\(y=\sqrt{\dfrac{2x+1}{x-3}}\)

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

=>\(y'=\dfrac{\dfrac{\left(2x+1\right)'\left(x-3\right)-\left(2x+1\right)\left(x-3\right)'}{\left(x-3\right)^2}}{2\sqrt{\dfrac{2x+1}{x-3}}}\)

=>\(y'=\dfrac{\dfrac{2\left(x-3\right)-2x-1}{\left(x-3\right)^2}}{2\sqrt{\dfrac{2x+1}{x-3}}}\)

\(=-\dfrac{\dfrac{7}{\left(x-3\right)^2}}{2\sqrt{\dfrac{2x+1}{x-3}}}< 0\forall x\) thỏa mãn ĐKXĐ, trừ x=-1/2 ra

=>Hàm số luôn đồng biến trên \(\left(3;+\infty\right);\left(-\infty;-\dfrac{1}{2}\right)\)

c:

ĐKXĐ: x>=-3

 \(y=\left(x+1\right)\sqrt{x+3}\)

=>\(y'=\left(x+1\right)'\cdot\sqrt{x+3}+\left(x+1\right)\cdot\sqrt{x+3}'\)

=>\(y'=\sqrt{x+3}+\left(x+1\right)\cdot\dfrac{\left(x+3\right)'}{2\sqrt{x+3}}\)

=>\(y'=\sqrt{x+3}+\dfrac{x+1}{2\sqrt{x+3}}\)

=>\(y'=\dfrac{2x+6+x+1}{2\sqrt{x+3}}=\dfrac{3x+7}{2\sqrt{x+3}}\)

Đặt y'>0

=>3x+7>0

=>x>-7/3

Kết hợp ĐKXĐ, ta được: x>-7/3

Đặt y'<0

3x+7<0

=>x<-7/3

Kết hợp ĐKXĐ, ta được: \(-3< x< -\dfrac{7}{3}\)

Vậy: Hàm số đồng biến trên \(\left(-\dfrac{7}{3};+\infty\right)\) và nghịch biến trên \(\left(-3;-\dfrac{7}{3}\right)\)

d: \(y=\dfrac{x-1}{x^2+1}\)(ĐKXĐ: \(x\in R\))

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

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

Đặt y'>0

=>\(-x^2+2x+1>0\)

=>\(1-\sqrt{2}< x< 1+\sqrt{2}\)

Đặt y'<0

=>\(-x^2+2x-1< 0\)

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

Vậy: hàm số đồng biến trên khoảng \(\left(1-\sqrt{2};1+\sqrt{2}\right)\)

hàm số nghịch biến trên khoảng \(\left(1+\sqrt{2};+\infty\right);\left(-\infty;1-\sqrt{2}\right)\)

a: \(y=\left(x-1\right)^3\)

=>\(y'=\left[\left(x-1\right)^3\right]'=3\left(x-1\right)^2\cdot\left(x-1\right)'\)

\(=3\left(x-1\right)^2\)

b: \(y=\left(x+2\right)\left(2x^2-3\right)\)

=>\(y'=\left(x+2\right)'\left(2x^2-3\right)+\left(x+2\right)\left(2x^2-3\right)'\)

=>\(y'=2x^2-3+2\left(x+2\right)\)

\(=2x^2+2x+1\)

c: \(y=\left(x-1\right)^2\left(x+2\right)\)

=>\(y=\left(x^2-2x+1\right)\left(x+2\right)\)

=>\(y'=\left(x^2-2x+1\right)'\left(x+2\right)-\left(x^2-2x+1\right)\left(x+2\right)'\)

=>\(y'=\left(2x-2\right)\left(x+2\right)-x^2+2x-1\)

\(=2x^2+4x-2x-4-x^2+2x-1\)

=>\(y'=x^2+4x-5\)

c: \(y=\left(x^2-1\right)\left(2x+1\right)\)

=>\(y'=\left(x^2-1\right)'\left(2x+1\right)+\left(x^2-1\right)\left(2x+1\right)'\)

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

\(=4x^2+2x+2x^2-2=6x^2+2x-2\)

a: \(y=\left(x+2\right)\left(2x^2-3\right)\)

=>\(y'=\left(x+2\right)'\left(2x^2-3\right)+\left(x+2\right)\left(2x^2-3\right)'\)

=>\(y'=2x^2-3+\left(x+2\right)\cdot2x\)

\(\Leftrightarrow y'=2x^2-3+2x^2+4x=4x^2+4x-3\)

b: \(y=\left(x-1\right)^2\left(x+2\right)\)

=>\(y=\left(x^2-2x+1\right)\left(x+2\right)\)

=>\(y'=\left(x^2-2x+1\right)'\left(x+2\right)+\left(x^2-2x+1\right)\left(x+2\right)'\)

=>\(y'=\left(2x-2\right)\left(x+2\right)+\left(x^2-2x+1\right)\)

=>\(y'=2x^2+4x-2x-4+x^2-2x+1\)

=>\(y'=3x^2-3\)

c: \(y=\left(x^2-1\right)\left(2x+1\right)\)

=>\(y'=\left(x^2-1\right)'\left(2x+1\right)+\left(x^2-1\right)\left(2x+1\right)'\)

=>\(y'=2x\left(2x+1\right)+2\left(x^2-1\right)\)

=>\(y'=4x^2+2x+2x^2-2=6x^2+2x-2\)

d: \(y=\left(x+2\right)\left(2x^2-5\right)\)

=>\(y'=\left(x+2\right)'\left(2x^2-5\right)+\left(x+2\right)\left(2x^2-5\right)'\)

=>\(y'=2x^2-5+2x\left(x+2\right)=4x^2+4x-5\)

a. Làm gọn 1 chút xíu:

\(y=\left(x^{11}+2x^7-3x^5-6x\right)\left(3x^7+6x^2-2\right)\)

\(y'=\left(11x^{10}+14x^6-15x^4-6\right)\left(3x^7+6x^2-2\right)+\left(21x^6+12x\right)\left(x^{11}+2x^7-3x^5-6x\right)\)

b.

 \(y'=5\left(x^4-\dfrac{2}{3x}\right)^4\left(4x^3+\dfrac{2}{3x^2}\right)\Rightarrow y'\left(10\right)=5\left(10^4-\dfrac{2}{30}\right)^4\left(4.10^3+\dfrac{2}{300}\right)=?\)

c.

\(y'=\dfrac{7}{\left(x+1\right)^2}\Rightarrow y'\left(4\right)=\dfrac{7}{25}\)

Bài 2: 

a) Ta có: \(\Delta=\left(m-1\right)^2-4\cdot1\cdot\left(-m^2-2\right)\)
\(=m^2-2m+1+4m^2+8\)

\(=5m^2-2m+9>0\forall m\)

Do đó, phương trình luôn có hai nghiệm phân biệt với mọi m

Bài 1:

ĐKXĐ \(2x\ne y\)

Đặt \(\dfrac{1}{2x-y}=a;x+3y=b\)

HPT trở thành

\(\left\{{}\begin{matrix}a+b=\dfrac{3}{2}\\4a-5b=-2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{3}{2}-b\\4\left(\dfrac{3}{2}-b\right)-5b=-2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{3}{2}-b\\6-9b=-2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}b=\dfrac{8}{9}\\a=\dfrac{11}{18}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x+3y=\dfrac{8}{9}\\2x-y=\dfrac{18}{11}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=2x-\dfrac{18}{11}\\x+3\left(2x-\dfrac{18}{11}\right)=\dfrac{8}{9}\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{82}{99}\\y=\dfrac{2}{99}\end{matrix}\right.\)

Tất cả các hpt này đều giải bằng PP đặt ẩn phụ

a) \(\begin{cases}2\left(x+y\right)+3\left(x-y\right)=4\\\left(x+y\right)+2\left(x-y\right)=5\end{cases}\)

Đặt \(x+y=a\) ; \(x-y=b\) ta được:

\(\begin{cases}2a+3b=4\\a+2b=5\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}2a+3b=4\\2a+4b=10\end{cases}\)\(\Leftrightarrow\) \(\begin{cases}-b=-6\\2a+4b=10\end{cases}\)

\(\Leftrightarrow\) \(\begin{cases}b=6\\2a+4.6=10\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}a=-7\\b=6\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}x+y=6-7\\x-y=6-7\end{cases}\)

\(\Leftrightarrow\) \(\begin{cases}x-7=-1\\6-y=-1\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}x=6\\y=-7\end{cases}\)

Lúc khác mình làm tiếp mấy câu kia

Tiếp nào!

b) \(\begin{cases}\dfrac{3}{x}-\dfrac{4}{y}=2\\\dfrac{4}{x}-\dfrac{5}{y}=3\end{cases}\) Đặt \(\dfrac{1}{x}=a\) ; \(\dfrac{1}{y}=b\) ta được:

\(\begin{cases}3a-4b=2\\4a-5b=3\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}12a-16b=8\\12a-15b=9\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}-1b=-1\\12a-15b=9\end{cases}\)

\(\Leftrightarrow\) \(\begin{cases}b=1\\a=2\end{cases}\)\(\Leftrightarrow\) \(\begin{cases}a=2\\b=1\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}\dfrac{1}{a}=2\\\dfrac{1}{b}=1\end{cases}\) \(\Leftrightarrow\) \(\begin{cases}a=\dfrac{1}{2}\\b=1\end{cases}\)

c) Làm tương tự thay \(\dfrac{1}{2x-y}=a\) ; \(\dfrac{1}{x+y}=b\)

a.

ĐKXĐ: \(\left\{{}\begin{matrix}x\ge2\\y\ge3\end{matrix}\right.\)

\(\left\{{}\begin{matrix}3\sqrt{x-2}+3\sqrt{y-3}=9\\2\sqrt{x-2}-3\sqrt{y-3}=-4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}3\sqrt{x-2}+3\sqrt{y-3}=9\\5\sqrt{x-2}=5\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}3\sqrt{x-2}+3\sqrt{y-3}=9\\\sqrt{x-2}=1\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}\sqrt{x-2}=1\\\sqrt{y-3}=2\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=7\end{matrix}\right.\)

b.

ĐKXĐ: \(\left\{{}\begin{matrix}x\ne-1\\y\ne-4\end{matrix}\right.\)

\(\left\{{}\begin{matrix}\dfrac{15x}{x+1}+\dfrac{10}{y+4}=20\\\dfrac{4x}{x+1}-\dfrac{10}{y+4}=8\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{15x}{x+1}+\dfrac{10}{y+4}=20\\\dfrac{19x}{x+1}=28\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{x}{x+1}=\dfrac{28}{19}\\\dfrac{1}{y+4}=-\dfrac{4}{19}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}19x=28x+28\\4y+16=-19\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=-\dfrac{28}{9}\\y=-\dfrac{35}{4}\end{matrix}\right.\)

a) Ta có: \(\left\{{}\begin{matrix}\sqrt{2}x-y=3\\x+\sqrt{2}y=\sqrt{2}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}\sqrt{2}x-y=3\\\sqrt{2}x+2y=2\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}-3y=1\\x+\sqrt{2}y=\sqrt{2}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{1}{3}\\x=\sqrt{2}-\sqrt{2}y\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{1}{3}\\x=\sqrt{2}-\sqrt{2}\cdot\dfrac{-1}{3}=\dfrac{4\sqrt{2}}{3}\end{matrix}\right.\)

Vậy: Hệ phương trình có nghiệm duy nhất là \(\left\{{}\begin{matrix}x=\dfrac{4\sqrt{2}}{3}\\y=-\dfrac{1}{3}\end{matrix}\right.\)

b) Ta có: \(\left\{{}\begin{matrix}\dfrac{x}{2}-2y=\dfrac{3}{4}\\2x+\dfrac{y}{3}=-\dfrac{1}{3}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}2x-8y=3\\2x+\dfrac{1}{3}y=-\dfrac{1}{3}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-\dfrac{25}{3}y=\dfrac{10}{3}\\2x-8y=3\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{2}{5}\\2x=3+8y=3+8\cdot\dfrac{-2}{5}=-\dfrac{1}{5}\end{matrix}\right.\)

hay \(\left\{{}\begin{matrix}x=-\dfrac{1}{10}\\y=-\dfrac{2}{5}\end{matrix}\right.\)

Vậy: Hệ phương trình có nghiệm duy nhất là \(\left\{{}\begin{matrix}x=-\dfrac{1}{10}\\y=-\dfrac{2}{5}\end{matrix}\right.\)

c) Ta có: \(\left\{{}\begin{matrix}\dfrac{2x-3y}{4}-\dfrac{x+y-1}{5}=2x-y-1\\\dfrac{x+y-1}{3}+\dfrac{4x-y-2}{4}=\dfrac{2x-y-3}{6}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{5\left(2x-3y\right)}{20}-\dfrac{4\left(x+y-1\right)}{20}=\dfrac{20\left(2x-y-1\right)}{20}\\\dfrac{4\left(x+y-1\right)}{12}+\dfrac{3\left(4x-y-2\right)}{12}=\dfrac{2\left(2x-y-3\right)}{12}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}10x-15y-4x-4y+4=40x-20y-20\\4x+4y-4+12x-3y-6=4x-2y-6\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}6x-19y+4-40x+20y+20=0\\16x+y-10-4x+2y+6=0\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}-34x+y=-24\\12x+3y=4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}-102x+3y=-72\\12x+3y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-114x=-76\\12x+3y=4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{2}{3}\\12\cdot\dfrac{2}{3}+3y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{2}{3}\\3y=4-8=-4\end{matrix}\right.\)

hay \(\left\{{}\begin{matrix}x=\dfrac{2}{3}\\y=-\dfrac{4}{3}\end{matrix}\right.\)

Vậy: Hệ phương trình có nghiệm duy nhất là \(\left\{{}\begin{matrix}x=\dfrac{2}{3}\\y=-\dfrac{4}{3}\end{matrix}\right.\)