\(A=6x^2+23x+21-\left(6x^2+23x-55\right)=76\\ B=x^4+x^3-x^2-2x^2-2x+2-x^4-x^3+3x^2+2x\\ =2\\ C=x^4+x^3-3x^2-2x-\left(x^4+x^3-x^2-2x^2-2x+2\right)\\ =-2\)

a: Ta có: \(3\left(2x-3\right)+2\left(2-x\right)=-3\)

\(\Leftrightarrow6x-9+4-2x=-3\)

\(\Leftrightarrow4x=2\)

hay \(x=\dfrac{1}{2}\)

Giúp mk vs

a: \(=x^3-2x^5\)

e: \(=x^4+2x^3-x^2-2x\)

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

\(\Rightarrow\left(x+3\right)\left(x^2-3x+5\right)=x\left(x+3\right)\)

\(\Rightarrow\left(x+3\right)\left(x^2-4x+5\right)=0\)

\(\Rightarrow x=-3\)(do \(x^2-4x+5=\left(x-2\right)^2+1\ge1>0\))

\(\Rightarrow\left(x+3\right)\left(x^2-3x+5\right)-x\left(x+3\right)=0\\ \Rightarrow\left(x+3\right)\left(x^2-4x+5\right)=0\\ \Rightarrow\left[{}\begin{matrix}x=-3\\\left(x-2\right)^2+1=0\left(vô.lí\right)\end{matrix}\right.\Rightarrow x=-3\)

Bài 5: 

a. 1 - 2y + y²

= (1 - y)²

b. (x + 1)² - 25

= (x + 1)² - 5²

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

= (x - 4)(x + 6)

c. 1 - 4x²

= 1² - (2x)²

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

d. 8 - 27x³

= 2³ - (3x)³

= (2 - 3x)(4 + 6x + 9x²)

e. (đề hơi khó hiểu ''x3'' !?)

g. x³ + 8y³

= (x + 2y)(x² - 2xy + y²)

A = - 3\(x\).(\(x-5\)) + 3(\(x^2\) - 4\(x\)) - 3\(x\) - 10

A = - 3\(x^2\) + 15\(x\) + 3\(x^2\) - 12\(x\) - 3\(x\) - 10

A = (- 3\(x^2\) + 3\(x^2\)) + (15\(x\) - 12\(x\) - 3\(x\)) - 10

A = 0 + (3\(x-3x\)) - 10

A = 0  - 10

A = - 10 

\(A=3x^3-6x^2+9x-3x^3+2x^2+5x^2-5x=x^2+4x\\ B=\left(x^2+xy+y^2\right)\left(x-y\right)=x^3-y^3\)

Lần sau bạn chú ý viết đầy đủ yêu cầu của đề bài.

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

b) \(B=-x^2+4x+5=-\left(x^2-4x-5\right)=-\left(x-5\right)\left(x+1\right)\)

c) \(C=9x^2-6x+3=3\left(3x^2-2x+1\right)\)

e. (2x - 5)² - (x + 3)² + 3x(x - 2)

= 4x² - 20x + 25 - x² - 6x - 9 + 3x² - 6x

= 6x² - 32x +16

f. (x + 3)(x² - 3x + 9) - x(x² + 5)

= x³ + 27 - x³ - 5x

= 27 - 5x