19

Từ pt đầu ta có:

\(x^2-xy-2xy+2y^2=0\)

\(\Leftrightarrow x\left(x-y\right)-2y\left(x-y\right)=0\)

\(\Leftrightarrow\left(x-y\right)\left(x-2y\right)=0\)

\(\Rightarrow\left[{}\begin{matrix}x=y\\x=2y\end{matrix}\right.\)

TH1: \(x=y\) thế xuống pt dưới:

\(y^2-y-y^2=1\Rightarrow y=-1\Rightarrow x=-1\)

TH2: \(x=2y\) thế xuống pt dưới:

\(\left(2y\right)^2-2y-y^2=1\Leftrightarrow3y^2-2y-1=0\)

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

Vậy nghiệm của hệ là: \(\left(x;y\right)=\left(-1;-1\right);\left(1;2\right);\left(-\dfrac{1}{3};-\dfrac{2}{3}\right)\)

21.

Từ pt đầu:

\(xy+2=2x+y\Leftrightarrow xy-y+2-2x=0\)

\(\Leftrightarrow y\left(x-1\right)-2\left(x-1\right)=0\)

\(\Leftrightarrow\left(x-1\right)\left(y-2\right)=0\)

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

TH1: \(x=1\) thế xuống pt dưới:

\(2y+y^2+3y=6\Leftrightarrow y^2+5y-6=0\)

\(\Rightarrow\left[{}\begin{matrix}y=1\\y=-6\end{matrix}\right.\)

TH2: \(y=2\) thế xuông pt dưới

\(4x+4+6=6\Rightarrow x=-1\)

Vậy nghiệm của pt là: \(\left(x;y\right)=\left(1;1\right);\left(1;-6\right);\left(-1;2\right)\)

Ai giúp em với ạ

Bài 3:

a: \(36x^2-49=0\)

=>\(36x^2=49\)

=>\(x^2=\frac{49}{36}\)

=>\(\left[\begin{array}{l}x=\frac76\\ x=-\frac76\end{array}\right.\)

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

=>\(\left(x-1\right)\left(x+2\right)-\left(x+2\right)=0\)

=>(x+2)(x-2)=0

=>\(\left[\begin{array}{l}x+2=0\\ x-2=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=-2\\ x=2\end{array}\right.\)

c: \(x^2\left(x+1\right)+2x\left(x+1\right)=0\)

=>\(\left(x+1\right)\left(x^2+2x\right)=0\)

=>x(x+1)(x+2)=0

=>\(\left[\begin{array}{l}x=0\\ x+1=0\\ x+2=0\end{array}\right.\Rightarrow\left[\begin{array}{l}x=0\\ x=-1\\ x=-2\end{array}\right.\)

d: x(2x-3)-3(3-2x)=0

=>x(2x-3)+3(2x-3)=0

=>(2x-3)(x+3)=0

=>\(\left[\begin{array}{l}2x-3=0\\ x+3=0\end{array}\right.\Rightarrow\left[\begin{array}{l}2x=3\\ x=-3\end{array}\right.\Rightarrow\left[\begin{array}{l}x=\frac32\\ x=-3\end{array}\right.\)

e: \(2x^3\left(2x-3\right)-x^2\left(4x^2-6x+2\right)=0\)

=>\(4x^4-6x^3-4x^4+6x^3-2x^2=0\)

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

=>\(x^2=0\)

=>x=0

g: \(\left(x-2\right)^2-\left(x+3\right)^2=5+4\left(x+1\right)\)

=>\(x^2-4x+4-x^2-6x-9=5+4x+4\)

=>-10x-5=4x+9

=>-14x=14

=>x=-1

Bài 4:

a: \(5a^2b+10ab^2-5ab=5ab\cdot a+5ba\cdot2b-5ab\cdot1\)

=5ab(a+2b-1)

b: \(4x^2+8xy-3x-6y\)

=4x(x+2y)-3(x+2y)

=(x+2y)(4x-3)

c: 10x(x-y)-6y(y-x)

=10x(x-y)+6y(x-y)

=(x-y)(10x+6y)

=2(5x+3y)(x-y)

d: \(3x^2-6xy+3y^2-12\)

\(=3\left(x^2-2xy+y^2-4\right)\)

\(=3\left\lbrack\left(x-y\right)^2-4\right\rbrack\)

=3(x-y-2)(x-y+2)

e: \(16-x^2+2xy-y^2\)

\(=16-\left(x^2-2xy+y^2\right)\)

\(=16-\left(x-y\right)^2=\left(4-x+y\right)\left(4+x-y\right)\)

f: \(3x^2-5x+2\)

\(=3x^2-3x-2x+2\)

=3x(x-1)-2(x-1)

=(x-1)(3x-2)

g: \(x^2+5x-6\)

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

=x(x+6)-(x+6)

=(x+6)(x-1)

h: \(x^2-4x-5\)

\(=x^2-5x+x-5\)

=x(x-5)+(x-5)

=(x-5)(x+1)

i: \(16x-5x^2-3\)

\(=-5x^2+15x+x-3\)

=-5x(x-3)+(x-3)

=(x-3)(-5x+1)

j: \(x^2+x-20\)

\(=x^2+5x-4x-20\)

=x(x+5)-4(x+5)

=(x+5)(x-4)

k: \(\left(x^2+9\right)^2-36x^2\)

\(=\left(x^2-6x+9\right)\left(x^2+6x+9\right)\)

\(=\left(x-3\right)^2\cdot\left(x+3\right)^2\)

l: \(x^6-x^4-4x^3+4x\)

\(=x^4\left(x^2-1\right)-4x\left(x^2-1\right)\)

\(=\left(x^2-1\right)\left(x^4-4x\right)=x\left(x^3-4\right)\left(x-1\right)\left(x+1\right)\)

3:

a: =>(x-2)(x+2)-3(x-2)^2=0

=>(x-2)(x+2-3x+6)=0

=>(x-2)(-2x+8)=0

=>x=2 hoặc x=4

b: =>15x+10=12x+1

=>3x=-9

=>x=-3

c; =>x-3=0 hoặc 2x+4=0

=>x=-2 hoặc x=3

d: =>2/3x-1-1/4x+1/4=3

=>5/12x-3/4=3

=>5/12x=15/4

=>x=9

2 should not go out for a walk

3 will quit it at once

4 must buy a smaller one

5 may be very worried

6 may find another one

7 can understand the lecture well

8 has to leave the building in order

9 ought to come to visit me more

10 must be very happy

Để A nguyên thì \(x-3\in\left\{-1;1;2;-2\right\}\)

hay \(x\in\left\{2;4;5;1\right\}\)

Đợi hết 5 phút đi r t giải cho =))

tế bào là cấu trúc cơ bản của mọi cơ thể 

hình dạng và kích thước rất nhỏ 

8)TẾ BÀO THỰC VẬT 

9)TẾ BÀO ĐỘNG VẬT 

còn mấy câu khác thì sao:)