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a) \(A=\left(x+1\right)\left(2x-1\right)\)
\(A=2x^2+2x-x-1\)
\(A=2x^2+x-1\)
\(A=2\left(x^2+\dfrac{1}{2}x-\dfrac{1}{2}\right)\)
\(A=2\left(x^2+2.x\dfrac{1}{4}+\dfrac{1}{16}-\dfrac{1}{16}-\dfrac{1}{2}\right)\)
\(A=2\left(x+\dfrac{1}{4}\right)^2-\dfrac{9}{8}\)
Vì \(2\left(x+\dfrac{1}{4}\right)^2\ge0\) với mọi x
\(\Rightarrow2\left(x+\dfrac{1}{4}\right)^2-\dfrac{9}{8}\ge-\dfrac{9}{8}\)
\(\Rightarrow Amin=-\dfrac{9}{8}\Leftrightarrow x=-\dfrac{1}{4}\)
\(B=4x^2-4xy+2y^2+1\)
\(B=\left(2x\right)^2-2.2x.y+y^2+y^2+1\)
\(B=\left(2x-y\right)^2+y^2+1\)
Vì \(\left(2x-y\right)^2\ge0\) với mọi x và y
\(y^2\ge0\) với mọi y
\(\Rightarrow\left(2x-y\right)^2+y^2+1\ge1\)
\(\Rightarrow Bmin=1\Leftrightarrow\left\{{}\begin{matrix}2x-y=0\\y=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}2x=0\\y=0\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=0\\y=0\end{matrix}\right.\)
\(C=5x-3x^2+2\)
\(C=-\left(3x^2-5x-2\right)\)
\(C=-3\left(x^2-\dfrac{5}{3}x-\dfrac{2}{3}\right)\)
\(C=-3\left(x^2-2.x.\dfrac{5}{6}+\dfrac{25}{36}-\dfrac{25}{36}-\dfrac{2}{3}\right)\)
\(C=-3\left(x-\dfrac{5}{6}\right)^2+\dfrac{49}{12}\)
Vì \(-3\left(x-\dfrac{5}{6}\right)^2\le0\) với mọi x
\(\Rightarrow-3\left(x-\dfrac{5}{6}\right)^2+\dfrac{49}{12}\le\dfrac{49}{12}\)
\(\Rightarrow Cmax=\dfrac{49}{12}\Leftrightarrow x=\dfrac{5}{6}\)
\(D=-8x^2+4xy-y^2+3\)
\(D=-\left(4x^2-4xy+y^2\right)-4x^2+3\)
\(D=-\left(2x-y\right)^2-4x^2+3\)
Vì \(-\left(2x-y\right)^2\le0\) với mọi x và y
\(-4x^2\le0\) với mọi x
\(\Rightarrow-\left(2x-y\right)^2-4x^2+3\le3\) với mọi x và y
\(\Rightarrow Dmax=3\Leftrightarrow\left\{{}\begin{matrix}x=0\\y=0\end{matrix}\right.\)
\(E=x^2-8x+38\)
\(E=x^2-2.x.4+16+22\)
\(E=\left(x-4\right)^2+22\)
Vì \(\left(x-4\right)^2\ge0\) với mọi x
\(\Rightarrow\left(x-4\right)^2+22\ge22\) với mọi x
\(\Rightarrow Emin=22\Leftrightarrow x=4\)
\(F=6x-x^2+1\)
\(F=-\left(x^2-6x-1\right)\)
\(F=-\left(x^2-2.x.3+9-9-1\right)\)
\(F=-\left(x-3\right)^2+10\)
Vì \(-\left(x-3\right)^2\le0\) với mọi x
\(\Rightarrow-\left(x-3\right)^2+10\le10\)
\(\Rightarrow Fmax=10\Leftrightarrow x=3\)
a, \(\left(2x+1\right)^2-2\left(2x+1\right)\left(x-3\right)+\left(x-3\right)^2\)
\(=\left(2x+1-x+3\right)^2=\left(x+4\right)^2\)
b, \(xy+xz+3y+3z=x\left(y+z\right)+3\left(y+z\right)=\left(x+3\right)\left(y+z\right)\)
c, \(xy-xz+y-z=x\left(y-z\right)+\left(y-z\right)=\left(x+1\right)\left(y-z\right)\)
d, \(x^2-xy-8x+8y=\left(x^2-xy\right)-\left(8x-8y\right)\)
\(=x\left(x-y\right)-8\left(x-y\right)=\left(x-8\right)\left(x-y\right)\)
e, \(x^2+2xy+y^2-xz-yz=\left(x^2+2xy+y^2\right)-\left(xz+yz\right)\)
\(=\left(x+y\right)^2-z\left(x+y\right)=\left(x+y+z\right)\left(x+y\right)\)
f, \(25-4x^2-4xy-y^2=25-\left(4x^2+4xy+y^2\right)\)
\(=5^2-\left(2x+y\right)^2=\left(5-2x-y\right)\left(5+2x+y\right)\)
1,
a, (2x + 1- x + 3)2 = (x+4)2
b,\(x\left(y+z\right)+3\left(y+z\right)=\left(y+z\right)\left(x+3\right)\)
c, \(x\left(y-z\right)+\left(y-z\right)=\left(y-z\right)\left(x+1\right)\)
d,\(x\left(x-y\right)+8\left(y-x\right)\)=\(\left(x-y\right)\left(x-8\right)\)
e,\(\left(x+y\right)^2-z\left(x+y\right)\)=\(\left(x+y\right)\left(x+y-z\right)\)
f,\(25-\left(4x^2+4xy+y^2\right)=5^2-\left(2x+y\right)^2\)
\(=\left(5+2x+y\right)\left(5-2x-y\right)\)
Chúc các bn hc tốt
B1 : a, M = x3-3xy(x-y)-y3-x2+2xy-y2
= ( x3-y3)-3xy(x-y) -(x2-2xy+y2)
= (x-y)(x2+xy+y2)-3xy(x-y)-(x-y)2
= (x-y) [(x2+xy+y2-3xy-(x-y)]
= (x-y)[(x2-2xy+y2)-(x-y)
= (x-y)[(x-y)2-(x-y)]
= (x-y)(x-y)(x-y-1)
= (x-y)2(x-y-1)
= 72(7-1) = 49 . 6= 294
N = x2(x+1)-y2(y-1)+xy-3xy(x-y+1)-95
= x3+x2-(y3-y2)+xy-(3x2y-3xy2+3xy)-95
= x3+x2-y3+y2+xy-3x2y+3xy2-3xy-95
= (x3-y3)+(x2-2xy+y2)-(3x2y+y2)-(3x2y-3xy2)-95
=(x-y)(x2+xy+y2)+(x-y)2-3xy(x-y)-95
= (x-y)(x2+xy+y2+x-y-3xy)-95
= (x-y)[(x2-2xy+y2)+(x-y)]-95
= (x-y)[(x-y)2+(x-y)]-95
=(x-y)(x-y)(x-y+1)-95
= (x-y)2(x-y+1)-95
= 72(7+1)-95=297
a, \(x^3-x^2y-xy^2+y^3\)
\(=x^2\left(x-y\right)-y^2\left(x-y\right)\)
\(=\left(x^2-y^2\right)\left(x-y\right)\)
\(=\left(x-y\right)^2\left(x+y\right)\)
b, \(x^3+x^2-4x-4\)
\(=x^2\left(x+1\right)-4\left(x+1\right)\)
\(=\left(x^2-4\right)\left(x+1\right)=\left(x-2\right)\left(x+2\right)\left(x+1\right)\)
c, \(x^3-x^2-x+1\)
\(=x^2\left(x-1\right)-\left(x-1\right)=\left(x^2-1\right)\left(x-1\right)\)
\(=\left(x-1\right)^2\left(x+1\right)\)
d, \(\left(7x+3\right)^2-\left(2x-1\right)^2\)
\(=\left(7x+3-2x+1\right)\left(7x+3+2x-1\right)\)
\(=\left(5x+4\right)\left(9x+2\right)\)
e, \(x^3-3x^2-3x+1\) sai đề
f, \(x^2-2x-3\)
\(=x^2-3x+x-3=x\left(x-3\right)+\left(x-3\right)\)
\(=\left(x+1\right)\left(x-3\right)\)
g, \(x^2-2x-8\)
\(=x^2-4x+2x-8=x\left(x-4\right)+2\left(x-8\right)\)
\(=\left(x+2\right)\left(x-8\right)\)
h, \(x^2-10x+21\)
\(=x^2-7x-3x+21\)
\(=x\left(x-7\right)-3\left(x-7\right)=\left(x-3\right)\left(x-7\right)\)
i, \(x^2-4xy+3y^2\)
\(=x^2-4xy+4y^2-y^2\)
\(=\left(x-2y\right)^2-y^2\)
\(=\left(x-2y-y\right)\left(x-2y+y\right)\)
\(=\left(x-3y\right)\left(x-y\right)\)
a) \(x^3 - x^2y - xy^2 + y^3\)
\(=\left(x^3-x^2y\right)-\left(xy^2-y^3\right)\)
\(=x^2\left(x-y\right)-y^2\left(x-y\right)\)
\(=\left(x-y\right)\left(x^2-y^2\right)\)
\(=\left(x-y\right)\left(x-y\right)\left(x+y\right)\)
\(=\left(x-y\right)^2\left(x+y\right)\)
b) \(x^3 + x^2 - 4x - 4\)
\(=\left(x^3+x^2\right)-\left(4x+4\right)\)
\(=x^2\left(x+1\right)-4\left(x+1\right)\)
\(=\left(x+1\right)\left(x^2-4\right)\)
\(=\left(x+1\right)\left(x^2-2^2\right)\)
\(=\left(x+1\right)\left(x+2\right)\left(x-2\right)\)