7x² + 6x = 5x

,=> 7x² + 6x - 5x = 0

=> 7x² + x = 0

=> x(7x + 1) = 0

=> x = 0 hoặc 7x + 1 =0

=> x = 0  hoặc x = -1/7

Vậy...

7x² + 6x = 5x

=> 7x² + 6x - 5x = 0

7x² + x = 0

x.(7x+1) = 0

=> x = 0

7x + 1 = 0 => 7x = -1 => x = -1/7

KL: x = 0 hoặc x = -1/7

ai trả lời nhanh giúp ik với vội lắm

\(a^2-ab+b^2\ge\frac{1}{3}.\left(a^2+ab+b^2\right)\)

\(\Rightarrow3a^2-3ab+3b^2\ge a^2+ab+b^2\)

\(\Rightarrow3a^2-3ab+3b^2-a^2-b^2-ab\ge0\)

\(\Rightarrow2a^2-4ab+2b^2\ge0\)

\(2.\left(a^2-2ab+b^2\right)\ge0\)

\(\Rightarrow\left(a-b\right)^2\ge0\)( BĐT luôn đúng )

Vậy \(a^2-ab+b^2\ge\frac{1}{3}.\left(a^2+ab+b^2\right)\)

Ta có :\(A=\frac{1}{ab}+\frac{1}{a^2+b^2}=\frac{1}{2ab}+\frac{1}{a^2+b^2}+\frac{3}{2ab}\)

           \(A\ge\frac{4}{2ab+a^2+b^2}+\frac{3}{2ab}\)

           \(A\ge\frac{4}{\left(a+b\right)^2}+\frac{3}{\frac{\left(a+b\right)^2}{2}}\)

         \(A\ge4+6=10\)

Dấu "=" xảy ra \(\Leftrightarrow\hept{\begin{cases}a^2+b^2=2ab\\a+b=1\end{cases}}\)\(\Leftrightarrow\hept{\begin{cases}a=b\\a+b=1\end{cases}}\Leftrightarrow a=b=\frac{1}{2}\)

Vậy Min A = 10 <=> a = b = 1/2

\(5x^2-3x\left(x-2\right)\)

\(=5x^2-3x^2+6x\)

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

\(-4x^2+2x-4x\left(x-5\right)\)

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

\(=-8x^2+22x\)

\(3x\left(x-5\right)-5x\left(x+7\right)\)

\(=3x^2-15x-5x^2+35x\)

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

Ta có :

\(\left(a+b\right)^2\)

\(=\left(a+b\right)\left(a+b\right)\)

\(=a^2+ab+ba+b^2\)

\(=a^2+2ab+b^2\)

Vậy \(\left(a+b\right)^2=a^2+2ab+b^2\)

B1:

a,\(\left(3x-2\right)\left(x-3\right)=3x^2-9x-2x+6=3x^2-11x+6\)

b,\(\left(2x+1\right)\left(x+3\right)=2x^2+6x+x+3=2x^2+7x+3\)

c,\(\left(x-3\right)\left(3x-1\right)=3x^2-x-9x+3=3x^2-10x+3\)

B2:

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

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

\(=x^2+2x-x^2-4x+2x+8=8\)

a)\(x^2-25y^2-xz-5yz\)

\(=\left(x-5y\right)\left(x+5y\right)-z\left(x+5y\right)\)

\(=\left(x-5y-z\right)\left(x+5y\right)\)

b)\(x^3-2x^2-4x+8\)

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

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

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

a) \(x^2-25y^2-xz-5yz=\left(x^2-25y^2\right)-\left(xz+5yz\right)\)

                                                   \(=\left(x+5y\right)\left(x-5y\right)-z\left(x+5y\right)\)

                                                     \(=\left(x+5y\right)\left(x-5y-z\right)\)

b)\(x^3-2x^2-4x+8=\left(x^3+8\right)-\left(2x^2+4x\right)\)

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

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

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

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

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

=\(3x^2\)-9x-4x+6

=\(3x^2\)+(-9x-4x)+6

=\(3x^2\)-13x+6

5) (2x+1)(x+3)=2x(x+3)+1(x+3)

=2x.x+2x.3+1.x+1.3

=\(2x^2\)+6x+1x+3

=\(2x^2\)+(6x+1x)+3

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

6) (x-3)(3x-1)=x(3x-1)-3(3x-1)

=x.3x+x.(-1)-3.3x-3.(-1)

=\(3x^2\)-1x-9x+3

=\(3x^2\)+(-1x-9x)+3

=\(3x^2\)-10x+3

rút gọn biểu thức

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

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

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

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

= -3x+4

B) x(x+2)-(x-2)(x+4)=x.x+x.2-x(x+4)+2(x+4)

=\(x^2+2x\)-x.x-x.4+2.x+2.4

=\(x^2+2x-x^2-4x+2x+8\)

=(\(x^2-x^2\))+(2x-4x+2x)+8

=8

tính giá trị biểu thức

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

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

=3x-6-2.x-2.(-3)-x.x-x(-3)

=3x-6-2x+6-\(x^2\)+3x

=(3x-2x+3x)+(-6+6)\(-x^2\)

=4x - \(x^2\)

thay x=-8 vào biểu thức thu gọn ta được:

4.(-8)- (-8)\(^2\)

= - 32 +64

= 32

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

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

=3x -\(x^2\)-1.1-1 .(-x)-x.1-x.(-x)

=3x\(-x^2\)-\(1^2\)+1x-1x+\(x^2\)

=(3x+1x-1x)+(\(-x^2+x^2\))-1

=3x-1

thay x=-5 vào biểu thức thu gọn ta được:

3.(-5)-1

=-15-1

=-16

Thu gọn biểu thức

4) (3x - 2) (x - 3) 

= ( 3x² - 2x ) - ( 3x x 3 - 2 x 3 )

= 3x² - 2x - 3x x 3 + 2 x 3

= 3x² - 2x - 9x + 6

= 3x² - 11x + 6 

5) (2x + 1) (x + 3) 

= ( 2x² + 1x ) + ( 6x + 3 )

= 2x² + 1x + 6x + 3

= 2x² + 7x + 3

6) (x - 3) (3x - 1) 

= ( 3x² - 9x ) - ( x - 3 )

= 3x² - 9x - x + 3

= 3x² - 10 + 3

Rút gọn biểu thức

A) x^2 - (x + 4) (x - 1)

= x² - ( x² + 4x ) - ( x + 4 )

= x² - x² - 4x - x - 4

= -5x - 4

B) x (x + 2) - (x - 2) (x + 4)

= x² + 2x - ( x² - 2x ) + ( 4x - 8 )

= x² + 2x - x² + 2x + 4x - 8

= 8x - 8

Tính giá trị biểu thức

A = 3 (x - 2) - (2 + x) (x - 3) tại x = - 8

Thế x = -8 vào, ta có :

= 3 ( -8 -2 ) - ( 2 + -8 ) ( -8 - 3 )

= 3 x ( -10 ) - ( - 6 ) ( -11 )

= -30 - 66

= -96

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

Thế x = - 5 vào, ta có :

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

= -5 x 8 - (-4) x 6

= - 40 - -24

= -40 + 24

= -16

100% đúng 

hok tốt nha 

A= (3x²+3y²⁾ - 2 (x+y) +6xy+1953

A= 3(x²+y²) - 2(x+y) +6xy+1953

A= 3(x²+2xy+y² - 2xy) - 2(x+y) +6xy +1953

A= 3 [(x+y)² -2xy] - 2(x+y) +6xy+1953

A= 3(x+y)² -6xy - 2(x+y) +6xy+1953

A= 3(x+y)² - 2(x+y)+1953

Thayy x+y =5 vào A, ta được: 

A= 3.5² - 2.5 +1953

A= 2018.