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\(A=x^3-y^3-21xy\)
\(A=\left(x-y\right).\left(x^2+xy+y^2\right)-21xy\)
\(A=7.\left(x^2+xy+y^2\right)-21xy\)
\(A=7.\left(x^2+xy+y^2+3xy\right)\)
\(A=7.\left(x^2+2xy+y^2+2xy\right)\)
\(A=7.\text{[}\left(x+y\right)^2+2xy\text{]}\)
\(A=7.\left(7^2+2xy\right)\)
\(A=7^3+14xy\)
Ngáo rồi @@
\(\)
\(A=x^3-y^3-21xy\)
\(\Rightarrow A=\left(x-y\right)\left(x^2+xy+y^2\right)-21xy\)
\(\Rightarrow A=7\left(x^2+xy+y^2\right)-21xy\)
\(\Rightarrow A=7\left(x^2+xy+y^2-3xy\right)\)
\(\Rightarrow A=7\left(x^2+y^2-2xy\right)\)
\(\Rightarrow A=7\left(x-y\right)^2\)
\(\Rightarrow A=7.7^2\)
\(\Rightarrow A=7.49\)
\(\Rightarrow A=343\)
a)
\(A=\left(x+3\right)\left(x^2-3x+9\right)-\left(54+x^3\right)\)
\(=x^3-3x^2+9x+3x^2-9x+27-54-x^3\)
\(=-27\)
or
\(A=x^3+27-54-x^3=-27\)
b)
\(B=\left(2x+y\right)\left(4x^2-2xy+y^2\right)-\left(2x-y\right)\left(4x^2+2xy+y^2\right)\)
\(=8x^3+y^3-8x^3+y^3=2y^3\)
c)
\(C=\left(2x+1\right)^2+\left(1-3x\right)^2+2\left(2x+1\right)\left(3x-1\right)\)
\(=\left(2x+1+3x-1\right)^2=\left(5x\right)^2=25x^2\)
d)
\(D=\left(x-2\right)\left(x^2+2x+4\right)-\left(x+1\right)^3+3\left(x-1\right)\left(x+1\right)\)
\(=x^3-8-\left(x-1\right)^3+3\left(x-1\right)\left(x+1\right)\)
\(=6x^2-3x-10\)
1.a, VT= \(\left(x^2+y^2\right)^2-\left(2xy\right)^2=\)\(\left(x^2+y^2-2xy\right)\left(x^2+y^2+2xy\right)=\left(x-y\right)^2\left(x+y\right)^2=VP.\left(đpcm\right)\)
b, VP=\(x\left(x-3y\right)^2+y\left(y-3x\right)^2\)\(=x\left(x^2-6xy+9y^2\right)+y\left(y^2-6xy+9x^2\right)\)\(=x^3-6x^2y+9xy^2+y^3-6xy^2+9x^2y\)
\(=x^3+3x^2y+3xy^2+y^3\)\(=\left(x+y\right)^3=VT\left(đpcm\right)\)
2. VT=\(\left(a+b\right)^3-\left(a-b\right)^3\)\(=\left(a+b-a+b\right)\left(a^2+2ab+b^2+a^2-b^2+a^2-2ab+b^2\right)\)
\(2b\left(b^2+3a^2\right)\)\(=VP\left(đpcm\right)\).
a) (x2 + y2)2 - (2xy)2
= [(x2 + y2) - 2xy].[(x2 + y2) + 2xy]
= [x2 + y2 - 2xy].[(x2 + y2 + 2xy]
= (x - y)2 . (x + y)2
Bài 3:
a: \(\Leftrightarrow M=6x^2+9xy-y^2-5x^2+2xy=x^2+11xy-y^2\)
b: \(\Leftrightarrow N=3xy-4y^2-x^2+7xy-8y^2=-x^2+10xy-12y^2\)
Bài 2:
\(A+B=4x^4-5xy+5y^2+3x^2+2xy-y=4x^4+3x^2-3xy+5y^2-y\)
\(A-B=4x^4-5xy+5y^2-3x^2-2xy+y=4x^4-3x^2+5y^2-7xy+y\)
\(B-A=-\left(A-B\right)=-4x^4+3x^2-5y^2+7xy-y\)
143. a) \(-6x^n.y^n.\left(-\dfrac{1}{18}x^{2-n}+\dfrac{1}{72}y^{5-n}\right)\)
\(=-6.\left(-\dfrac{1}{18}\right)x^n.x^{2-n}.y^n+\left(-6\right).\dfrac{1}{27}x^n.y^n.y^{5-n}\)
\(=\dfrac{1}{3}x^{n+2-n}y^n-\dfrac{2}{9}x^n.y^{n+5-n}\)
\(=\dfrac{1}{3}x^2y^n-\dfrac{2}{9}x^ny^5\)
b) Ta có: \(\left(5x^2-2y^2-2xy\right)\left(-xy-x^2+7y^2\right)\)
\(=5x^2\left(-xy\right)+5x^2.\left(-x^2\right)+5x^2.7y^2-2y^2.\left(-xy\right)-2y^2.\left(-x^2\right)-2y^2.7y^2-2xy.\left(-xy\right)-2xy\left(-x^2\right)-2xy.7y^2\)
\(=-5x^3y-5x^4+35x^2y^2+2xy^3+2x^2y^2-14y^4+2x^2y^2+2x^3y-14xy^3\)
Rút gọn các đa thức đồng dạng, ta có kết quả:
\(-5x^4-3x^3y+39x^2y^2-12xy^3-14y^4\)
Kết quả đã được xếp theo lũy thừa giảm dần của x
a) theo tính chất của dãy tỉ số bằng nhau có
\(\frac{x-y-z}{x}=\frac{-x+y-z}{y}=\frac{-x-y+z}{z}=\frac{x-y-z-x+y-z-x-y+z}{x+y+z}=\frac{-\left(x+y+z\right)}{x+y+z}=-1\)
=> x - y - z = - x => 2.x = y + z
y - x - z = - y => 2.y = x+z
z - x - y = - z => 2.z = x+y
Ta có: \(A=\left(1+\frac{y}{x}\right)\left(1+\frac{z}{y}\right)\left(1+\frac{x}{z}\right)=\frac{x+y}{x}.\frac{y+z}{y}.\frac{z+x}{z}=\frac{2z}{x}.\frac{2x}{y}.\frac{2y}{z}=\frac{2xyz}{xyz}=2\)
b) Vì \(\left|x+3y-1\right|\ge0\); \(-3\left|y+3\right|\le0\)
=> \(\left|x+3y-1\right|=-3\left|y+3\right|\) khi \(\left|x+3y-1\right|=-3\left|y+3\right|=0\)
=> x+ 3y - 1 = 0 và y + 3 = 0
=> x = 1 - 3y và y = -3 => x = 1- 3(-3) = 10; y = -3
=> C = 4.102.(-3) + 2.10.(-3)2 - (-3)2 = -1029
\(\left(x-3\right)^2+\left(y+2\right)^2=0\)
\(\left\{{}\begin{matrix}\left(x-3\right)^2\ge0\forall x\\\left(y+2\right)^2\ge0\forall y\end{matrix}\right.\)
\(\Rightarrow\left(x-3\right)^2+\left(y+2\right)^2\ge0\)
Dấu "=" xảy ra khi:
\(\left\{{}\begin{matrix}\left(x-3\right)^2=0\Rightarrow x-3=0\Rightarrow x=3\\\left(y+2\right)^2=0\Rightarrow y+2=0\Rightarrow y=-2\end{matrix}\right.\)
đề sai câu b các câu sau áp dụng tương tự
c/ Vì: \(\left(x-12+y\right)^{200}+\left(x-4-x\right)^{200}=0\)
mà \(\left\{{}\begin{matrix}\left(x-12+y\right)^{200}\ge0\forall x,y\\\left(x-4-y\right)^{200}\ge0\forall x,y\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}\left(x-12+y\right)^{200}=0\\\left(x-4-y\right)^{200}=0\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x-12+y=0\\x-4-y=0\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}x+y=12\\x-y=4\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=8\\y=4\end{matrix}\right.\)
\(\Rightarrow\sqrt{y\left(2x-y\right)}.\sqrt{z\left(2y-z\right)}.\sqrt{x\left(2z-x\right)}=xyz\)
\(\Rightarrow\sqrt{xyz}.\sqrt{\left(2x-y\right)\left(2y-z\right)\left(2z-x\right)}=xyz\)
\(\Rightarrow\sqrt{\left(2x-y\right)\left(2y-z\right)\left(2z-x\right)}=\sqrt{xyz}\)
=>(2x-y)(2y-z)(2z-x)=xyz
=>(2x-y)2(2y-z)2(2z-x)2=x2y2z2
=>8(2x-y)2(2y-z)2(2z-x)2=8x2y2z2
(3-x2)(3-y2)(3-z2)
=3x2y2+3y2z2+3z2x2-x2y2z2
sau đó phân tích cái 8(2x-y)2(2y-z)2(2z-x)2
\(\Rightarrow\sqrt{y\left(2x-y\right)}.\sqrt{z\left(2y-z\right)}.\sqrt{x\left(2z-x\right)}=xyz\)
\(\Rightarrow\sqrt{xyz}.\sqrt{\left(2x-y\right)\left(2y-z\right)\left(2z-x\right)}=xyz\)
\(\Rightarrow\sqrt{\left(2x-y\right)\left(2y-z\right)\left(2z-x\right)}=\sqrt{xyz}\)
=>(2x-y)(2y-z)(2z-x)=xyz
=>(2x-y)2(2y-z)2(2z-x)2=x2y2z2
=>8(2x-y)2(2y-z)2(2z-x)2=8x2y2z2
(3-x2)(3-y2)(3-z2)
=3x2y2+3y2z2+3z2x2-x2y2z2
sau đó phân tích cái 8(2x-y)2(2y-z)2(2z-x)2
a)Ta có:
\(\begin{cases} (x-2)^2 \geq 0 \\ (y-3)^2 \geq 0 \end{cases}\\ \Rightarrow (x-2)^2+(y-3)^2 =0 \Leftrightarrow \begin{cases} (x-2)^2 = 0 \Leftrightarrow x=2\\ (y-3)^2 = 0 \Leftrightarrow y=3 \end{cases}\\\)
Vậy x=2, y=3
b)
\(5^{(x-2).(x+3)}=1\\ \Leftrightarrow (x-2)(x+3)=0\\ \Rightarrow \left[ \begin{array}{} x-2=0 \Rightarrow x=2\\ x+3=0 \Rightarrow x=-3 \end{array}\right.\)
vậy x=2 hoặc -3
Ta có :
\(\left(x-2\right)^2\) luôn luôn lớn hơn hoặc bằng 0, \(\left(y-3\right)^2\) cũng luôn luôn lớn hơn bằng 0
để \(\left(x-2\right)^2+\left(y-3\right)^2=0\) thì \(\left(x-2\right)^2=\left(y-3\right)^2=0\)
hay x-2=0 => x=2
y-3=0 => y=3
Vậy cặp số (x,y) lần lượt bằng 2;3
\(M=2\left(x^3-y^3\right)-3\left(x^2+2xy+y^2\right)\)
\(=2\left[\left(x-y\right)^3+3xy\left(x-y\right)\right]-3\left(x^2-2xy+y^2+4xy\right)\)
\(=2.\left[\left(x-y\right)^3+3xy\left(x-y\right)\right]-3\left[\left(x-y\right)^2+4xy\right]\)
Thay \(x-y=2\)vào biểu thức ta được:
\(M=2.\left(2^3+3xy.2\right)-3\left(2^2+4xy\right)=2.\left(8+6xy\right)-3.\left(4+4xy\right)\)
\(=16+12xy-12-12xy=4\)