1) Tìm x biết : a) \(a^2x+x=2a^2-3\) ; b) \(a^2x+3ax+9=a^2\left(a\ne0;a\ne-3\right)\)
2) Cho a + b + c = 3,rút gọn biểu thức \(\frac{a^3+b^3+c^3-3abc}{\left(a-b\right)^3+\left(b-c\right)^3+\left(c-a\right)^3}\)
3) Chứng minh rằng nếu \(\frac{1}{x}-\frac{1}{y}-\frac{1}{z}=1;x=y+z\)thì \(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}=1\)
b. Sử dụng các hằng đẳng thức
\(a^3+b^3+c^2-3abc=\left(a+b+c\right)\left(a^2+b^2+c^2-ab-bc-ca\right)\)
\(=3\left(a^2+b^2+c^2-ab-bc-ca\right)\)
và \(\left(a-b\right)^3+\left(b-c\right)^3+\left(c-a\right)^3=3\left(a-b\right)\left(b-c\right)\left(c-a\right)\)
nên \(A=\frac{a^2+b^2+c^2-ab-bc-ca}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}=\frac{1}{2}.\frac{\left[\left(a-b\right)^2+\left(b-c\right)^2+\left(c-a\right)^2\right]}{\left(a-b\right)\left(b-c\right)\left(c-a\right)}\)
Do (a - b) + (b - c) + (c - a) = 0 nên áp dụng hđt \(X^2+Y^2+Z^2=-2\left(XY+YZ+ZX\right)\)khi X + Y + Z = 0, ta có:
\(A=-2\left(\frac{1}{a-b}+\frac{1}{b-c}+\frac{1}{c-a}\right).\)
Bài 1 :
\(b,ax^2+3ax+9=a^2\)
\(\Leftrightarrow a^2x+3ax+9-a^2=0\)
\(\Leftrightarrow ax\left(a+3\right)+\left(a+3\right)\left(3-a\right)=0\)
\(\Leftrightarrow\left(a+3\right)\left(ax+3-a\right)=0\)
Vì \(a\ne3\Rightarrow\left(a+3\right)\ne0\Rightarrow ax+3-a=0\)
\(\Leftrightarrow ax=a-3\)
Vì \(a\ne0\Rightarrow x=\frac{a-3}{a}\)
c.Ta có \(\left(\frac{1}{x}-\frac{1}{y}-\frac{1}{z}\right)^2=\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}-\frac{2}{xz}-\frac{2}{xy}+\frac{2}{yz}=1\)
Do x = y + z nên \(\frac{-2}{xz}-\frac{2}{xy}+\frac{2}{yz}=\frac{-2y-2z+2\left(y+z\right)}{\left(y+z\right)zy}=0\)
Vậy nên \(\frac{1}{x^2}+\frac{1}{y^2}+\frac{1}{z^2}=1.\)
Cô sửa KL \(a=-\left(\frac{1}{a-b}+\frac{1}{b-c}+\frac{1}{c-a}\right)=\frac{1}{b-a}+\frac{1}{c-b}+\frac{1}{a-c}\)
