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\(A=-4x^2-5y^2+8xy+10y+12\)
\(-A=4x^2+5y^2-8xy-10y-12\)
\(-A=\left(4x^2-8xy+y^2\right)+\left(4y^2-10y+\frac{25}{4}\right)-\frac{73}{4}\)
\(-A=\left(2x-y\right)^2+\left(2y-\frac{5}{2}\right)^2-\frac{73}{4}\)
Mà : \(\left(2x-y\right)^2\ge0\forall x;y\)
\(\left(2y-\frac{5}{2}\right)^2\ge0\forall y\)
\(\Rightarrow-A\ge-\frac{73}{4}\)
\(\Leftrightarrow A\le\frac{73}{4}\)
Dấu "=" xảy ra khi :
\(\hept{\begin{cases}2x-y=0\\2y-\frac{5}{2}=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=\frac{5}{8}\\y=\frac{5}{4}\end{cases}}\)
Vậy \(A_{Max}=\frac{73}{4}\Leftrightarrow\left(x;y\right)=\left(\frac{5}{8};\frac{5}{4}\right)\)
a) 4x2 - 5xy + y2 = 4x2 - 4xy - xy + y2 = 4x( x - y ) - y( x - y ) = ( x - y )( 4x - y )
b) x2 - 4xy + 3y2 = x2 - xy - 3xy + 3y2 = x( x - y ) - 3y( x - y ) = ( x - y )( x - 3y )
c) 9x2 + 6xy - 8y2 = 9x2 - 6xy + 12xy - 8y2 = 9x( x - 2/3y ) + 12y( x - 2/3y ) = ( x - 2/3y )( 9x + 12y )
d) 2x2 + 3xy - 5y2 = 2x2 - 2xy + 5xy - 5y2 = 2x( x - y ) + 5y( x - y ) = ( x - y )( 2x + 5y )
e) x2 - 35y2 - 2xy = x2 + 5xy - 7xy - 35y2 = x( x + 5y ) - 7y( x + 5y ) = ( x + 5y )( x - 7y )
f) 2x2 + 10xy + 8y2 = 2( x2 + 5xy + 4y2 ) = 2( x2 + xy + 4xy + 4y2 ) = 2[ x( x + y ) + 4y( x + y ) ] = 2( x + y )( x + 4y )
g) x2 - 10xy + 16y2 = x2 - 2xy - 8xy + 16y2 = x( x - 2y ) - 8y( x - 2y ) = ( x - 2y )( x - 8y )
h) 4x2 + 4xy - 15y2 = 4x2 - 6xy + 10xy - 15y2 = 4x( x - 3/2y ) + 10y( x - 2/3y ) = ( x - 2/3y )( 4x + 10y )
i) -7xy + 3x2 + 2y2 = 3x2 - xy - 6xy + 2y2 = 3x( x - 1/3y ) - 6y( x - 1/3y ) = ( x - 1/3y )( 3x - 6y )
j) 56y2 + 4x2 - 36xy = 4( x2 - 9xy + 14y2 ) = 4( x2 - 2xy - 7xy + 14y2 ) = 4[ x( x - 2y ) - 7y( x - 2y ) ] = 4( x - 2y )( x - 7y )
a) 5x2 - 10x = 5x( x - 2 )
b) x2 - y2 - 2x + 2y = (x2 - y2) - (2x - 2y)
= (x - y ) ( x + y)-2 (x-y)
= ( x - y) ( x + y - 2)
c) 4x2 - 4xy - 8y2 = (4x2 - 4xy + 8y2) - 9y2
= (2x - 9y2) - 3y2
= (2x - y - 3y) (2x - y + 3y)
= (2x - 4y) (2x + 2y)
= 4(x - 2y) (x + y)
a) 5x2 - 10x = 5x( x - 2 )
b) x2 - y2 - 2x + 2y = (x2 - y2) - (2x - 2y)
= (x - y ) ( x + y)-2 (x-y)
= ( x - y) ( x + y - 2)
c) 4x2 - 4xy - 8y2 = (4x2 - 4xy + 8y2) - 9y2
= (2x - 9y2) - 3y2
= (2x - y - 3y) (2x - y + 3y)
= (2x - 4y) (2x + 2y)
= 4(x - 2y) (x + y)
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1 2y(4X bình -9)
2 5(X bình +10+1-Y bình)
3 3X(X bình +2X+1-4Y bình)
4 ab(a bình-b bình) + (a+b)bình
5 2X bình (X-4) - 2Xy(Y-4)
xong
\(8x^2-18y=2y\left(4x^2-9\right)\)
5)\(2x^3-2xy^2-8x^2+8xy\)=\(2x\left(x^2-Y^2\right)-8X\left(x+y\right)\)=\(2x\left(x+y\right)\left(x-y\right)-8x\left(x+y\right)\)=
\(a,6x^3-9x^2=3x^2\left(2x-3\right)\)
\(b,4x^2y-8xy^2+10x^2y^2=2xy\left(2x-4y+5xy\right)\)
\(c,20x^2y-12x^3=4x^2\left(5y-3x\right)\)
\(d,4xy^2+8xyz=4xy\left(y+2z\right)\)
\(x^2+8xy+16y^2+2x+8y-3\)
\(=x^2+2.x.4y+\left(4y\right)^2+2\left(x+4y\right)-3\)
\(=\left(x+4y\right)^2+2\left(x+4y\right)+1-2^2\)
\(=\left(x+4y+1\right)^2-2^2\)
\(=\left(x+4y+1-2\right)\left(x+4y+1+2\right)\)
\(=\left(x+4y-1\right)\left(x+4y+3\right)\)
\(4x^2+4xy+y^2+10x+5y-6\)
\(=\left(2x\right)^2+2.2x.y+y^2+5\left(2x+y\right)-6\)
\(=\left(2x+y\right)^2+5\left(2x+y\right)-6\)
\(=\left(2x+y\right)^2+2\left(2x+y\right).\dfrac{5}{2}+\left(\dfrac{5}{2}\right)^2-\left(\dfrac{7}{2}\right)^2\)
\(=\left(2x+y+\dfrac{5}{2}\right)^2-\left(\dfrac{7}{2}\right)^2\)
\(=\left(2x+y+\dfrac{5}{2}-\dfrac{7}{2}\right)\left(2x+y+\dfrac{5}{2}+\dfrac{7}{2}\right)\)
\(=\left(2x+y-1\right)\left(2x+y+6\right)\)