Giá trị của bt A=3x^2-2(x-y)^2-3y^2 tại     x=4  y=-4

a/ A=3x^2−2(x−y)^2−3y^2

=3(x2−y2)−2(x−y)^2

=3(x−y)(x+y)−2(x−y)^2

=(x−y)(3x+3y−2x+2y)

=(x−y)(x+5y)

Thay x=4;y=−4vào biểu thức ta có :

A=(4−(−4))(4+5.(−4))=−128

Vậy...

giúp nhanh mn

đang cần gấp 

\(a^2-2ab+b^2-c^2\)

\(=\left(a-b\right)^2-c^2\)

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

6x^2 - xy - 7y^2 = -(y + x)(7y - 6x)

nha bạn chúc bạn học tốt 

6x^2-xy-7y^2

=6x^2+6xy-7xy-7y^2

=6x(x+y)-7y(x+y)

=(6x-7y)(x+y)

HT

\(x^2-11x+28\)

\(=x^2-4x-7x+28=x\left(x-4\right)-7\left(x-4\right)=\left(x-7\right)\left(x-4\right)\)

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

\(\left(a^2+5\right)\left(5-a^2\right)=25-a^4\)

( 2x + 3 ) ( 2x - 3 )

= [ ( 2x )² + 6x ] - ( 6x + 9 )

= ( 2x )² + ( 6x - 6x ) + 9

= ( 2x )² + 9

\(\frac{1}{x+2}+\frac{1-x}{x-2}=\frac{x}{x^2-4}\)ĐK : \(x\ne\pm2\)

\(\Leftrightarrow\frac{x-2+\left(1-x\right)\left(x+2\right)}{\left(x-2\right)\left(x+2\right)}=\frac{x}{\left(x-2\right)\left(x+2\right)}\)

\(\Rightarrow x-2+x+2-x^2-2x=x\Leftrightarrow-x^2-x=0\Leftrightarrow-x\left(x+1\right)=0\Leftrightarrow x=0;x=-1\)

\(1+6y+12y^2+8y^3=1^3+3.1^3.2y+3.1.\left(2y\right)^2+\left(8y\right)^3=\left(1+2y\right)^3\)

4x^2 - 8x + 9

= (2x^2) - 2.2x.2 + 4 + 5

= (2x - 2)^2 + 5

có (2x - 2)^2 ≥ 0 => (2x-2)^2 + 5 ≥ 5

dấu = xảy ra <=> 2x - 2 = 0<=> x = 1

vậy min = 5 khi x = 1

\(4x^2-8x+9\)

\(=2x^2-2\times2x\times2+4+5\)

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

Ta có:

\(\left(2x-2\right)^2\ge0\)

\(\Rightarrow\left(2x-2\right)^2+5\ge5\)

Dấu bằng xảy ra khi:

\(2x-2=0\)

\(\Rightarrow x=1\)

Vậy min = \(5\)khi \(x=1\)

#Hok tốt#

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