B1
$3x^{2}$($ax^{2}$ - $2bx$ - $3c$) = $3x^{4}$ - $12x^{3}$ + $27x^{2}$
$\Rightarrow$ $3ax^{4}$ $-$ $6bx^{3}$ $-$ $9cx^{2}$ = $3x^{4}$ - $12x^{3}$ + $27x^{2}$
$\Rightarrow$ $\begin{cases} a=1\\b=2\\c=-3 \end{cases}$
B2
Ta có VP=4m(m-a) = 4m² - 4am = (2m)² - 2a(2m)
=(a+b+c)² - 2a(a+b+c) = (a+b+c)(a+b+c - 2a)
=(a+b+c)(b+c-a) = ab + ac - a² +b² + bc - ab +bc +c² -ac
=2bc +b² +c² - a² = VT
