\(a,\left(x-1\right)\left(x+2\right)\left(x+3\right)\left(x+6\right)-28=0\)
\(\Leftrightarrow\left[\left(x-1\right)\left(x+6\right)\right]\left[\left(x+2\right)\left(x+3\right)\right]-28=0\)
\(\Leftrightarrow\left(x^2+5x-6\right)\left(x^2+5x+6\right)-28=0\)
\(\Leftrightarrow\left(x^2+5x\right)^2-36-28=0\)
\(\Leftrightarrow\left(x^2+5x\right)^2-64=0\)
\(\Leftrightarrow\left(x^2+5x-8\right)\left(x^2+5x+8\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\frac{-\sqrt{57}}{2}-\frac{5}{2}\\x=\frac{\sqrt{57}}{2}-\frac{5}{2}\end{matrix}\right.\)
b, \(\left(x^2+4x+3\right)\left(x^2+6x+8\right)=0\)
\(\Leftrightarrow\left(x^2+3x+x+3\right)\left(x^2+4x+2x+8\right)=0\)
\(\Leftrightarrow\left(x+3\right)\left(x+1\right)\left(x+4\right)\left(x+2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-3\\x=-1\\x=-2\\x=-4\end{matrix}\right.\)
\(\left(a-1\right)\left(a+2\right)\left(a+3\right)\left(a+6\right)-28=\left(a-1\right)\left(a+6\right)\left(a+2\right)\left(a+3\right)-28=\left(a^2+5a-6\right)\left(a^2+5a+6\right)-28=\left(a^2+5a\right)^2-36-28=\left(a^2+5a\right)^2=64\Leftrightarrow a^2+5a=\pm8;a^2+5a+6,25=\left(a+2,5\right)^2\ge0\Rightarrow a^2+5a\ge-6,25\Rightarrow a^2+5a=8\Leftrightarrow\left(a+2,5\right)^2=14,25\Leftrightarrow a=\pm\sqrt{14,25}-2,5\)
