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\(\left(a^2+b^2+c^2\right)^2-a^2b^2-b^2c^2-a^2c^2\)
\(=a^4+b^4+a^2b^2+2a^2b^2+2a^3b+2ab^3-a^2b^2-b^2c^2-c^2a^2\)
\(=\left(a^4+2a^2b^2+b^4\right)+\left(2a^3b+2ab^3\right)-\left(a^2c^2+b^2c^2\right)\)
\(=\left(a^2+b^2\right)^2+2ab.\left(a^2+b^2\right)-c^2.\left(a^2+b^2\right)\)
\(=\left(a^2+b^2\right).\left(a^2+b^2+2ab-c^2\right)\)
\(=\left(a^2+b^2\right).\left[\left(a+b\right)^2-c^2\right]\)
\(=\left(a^2+b^2\right).\left(a+b-c\right).\left(a+b+c\right)\)
(a+b+c)(ab+bc+ca)−abc
=(a+b)(ab+bc+ac)+c(ab+bc+ca)−abc
=(a+b)(ab+bc+ca)+abc+c2(a+b)−abc
=(a+b)(ab+bc+ca+c2)
=(a+b)(b+c)(c+a)
nguồn: https://h7.net/hoi-dap/toan-8/phan-h-a-b-c-ab-bc-ca-abc-thanh-nhan-tu--faq429360.html
Bài 4:
Ta có: \(\left(x^3-x^2\right)-4x^2+8x-4=0\)
\(\Leftrightarrow x^2\left(x-1\right)-4\left(x-1\right)^2=0\)
\(\Leftrightarrow\left(x-1\right)\left(x-2\right)^2=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=1\\x=2\end{matrix}\right.\)
Ta có:A= 3a+3b-a^2-ab
=>A= (3a-a^2)+(3b-ab)
=>A= a(3-a)+b(3-a)
=>A= (a+b)(3-a)
\(ab\left(b-a\right)-bc\left(b-c\right)-ac\left(c-a\right)\)
\(=ab\left(b-a\right)-b^2c+bc^2-ac^2+a^2c\)
\(=ab\left(b-a\right)+c^2\left(b-a\right)-c\left(b^2-a^2\right)\)
\(=\left(b-a\right)\left(ab+c^2-bc-ca\right)\)
\(=\left(b-a\right)\left[b\left(a-c\right)-c\left(a-c\right)\right]\)
\(=\left(b-a\right)\left(a-c\right)\left(b-c\right)\)
\(a^2-ab+a-b\)
\(=a\left(a-b\right)+\left(a-b\right)\)
\(=\left(a-b\right)\left(a+1\right)\)
a2-ab+a-b
=a(a-b)+(a-b)
=(a+1)(a-b)