B) 7a² - 7ax - 9a + 9x

=(7a2-7ax)-(9a-9x)

=7a(a-x)-9(a-x)

=(a-x)(7a-9)

C) xa - xb+ 3a - 3b

=(xa-xb)+(3a-3b)

=x(a-b)+3(a-b)

=(a-b)(x+3)

A) x² - x- y² - y

=(x²-y²)-(x+y)

=(x-y)(x+y)-(x+y)

=(x+y)(x-y-1)

B) x² - 2xy + y² - z²

= (x²-2xy+y²)-z²

=(x-y)²-z²

=(x-y+z)(x-y-z)

C) 5x - 5y + ax - ay

= (5x-5y)+(ax-ay)

=5(x-y)+a(x-y)

= (x-y)(5+a)

. 4x²-y²+1-4x

=(4x²-4x+1)-y²

=(2x-1)²-y²

=(2x-1-y)(2x-1+y)

2. 6x²-7x-20

=6x²+8x-15x-20

=2x(3x+4)-5(3x+4)

=(2x-5)(3x+4)

8,2 tạ = .820.

16m 75mm=.......16,075...m

\(\dfrac{3}{4}\) m=\(\dfrac{15}{2}\) dm

bà chị vẽ hình cho cáiKhánh Huyền

b) \(\dfrac{x+1}{x-3}-\dfrac{1-x}{x+3}-\dfrac{2x\left(1-x\right)}{9-x^2}\)

=\(\dfrac{x+1}{x-3}-\dfrac{1-x}{x+3}+\dfrac{2x\left(1-x\right)}{x^2-9}\)

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

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

=\(\dfrac{x^2+3x+x+3-\left(x-3-x^2+3x\right)+2x-2x^2}{\left(x-3\right)\left(x+3\right)}\)

=\(\dfrac{x^2+4x+3-x+3+x^2-3x+2x-2x^2}{\left(x-3\right)\left(x+3\right)}\)

=\(\dfrac{2x+6}{\left(x-3\right)\left(x+3\right)}=\dfrac{2\left(x+3\right)}{\left(x-3\right)\left(x+3\right)}=\dfrac{2}{x-3}\)

5.\(C\text{ó}x^2-12=0\Rightarrow x^2=12\Rightarrow x=\sqrt{12}ho\text{ặc}x=-\sqrt{12}\)

Mà x>0\(\Rightarrow x=\sqrt{12}\)

6.Vì x-y=4\(\Rightarrow\left(x-y\right)^2=x^2-2xy+y^2=x^2-10+y^2=4^2=16\Rightarrow x^2+y^2=26\)

Có \(\left(x+y\right)^2=x^2+2xy+y^2=26+10=36=6^2=\left(-6\right)^2\)

Vì xy>0 và x>0 =>y>0=>x+y>0=>x+y=6

7. \(3x^2+7=\left(x+2\right)\left(3x+1\right)\)

\(3x^2+7=3x^2+7x+2\)

\(3x^2+7-3x^2-7x-2=0\)

-7x+5=0

-7x=-5

\(x=\frac{5}{7}\)

8.\(\left(2x+1\right)^2-4\left(x+2\right)^2=9\)

\(\left(2x+1\right)^2-\left(2x+4\right)^2=9\)

(2x+1-2x-4)(2x+1+2x+4)=9

-3(4x+5)=9

4x+5=-3

4x=-8

x=-2

Còn câu 9 và 10 để mình nghiên cứu đã