\(a,ĐK:x\ge-5\\ A=2\sqrt{x+5}-2\sqrt{x+5}+3\sqrt{x+5}=3\sqrt{x+5}\\ b,A=6\Leftrightarrow\sqrt{x+5}=\dfrac{6}{3}=2\\ \Leftrightarrow x+5=4\\ \Leftrightarrow x=-1\left(tm\right)\)

\(A=2\sqrt{x+5}-2\sqrt{x+5}+3\sqrt{x+5}\)

\(A=3\sqrt{x+5}\)

\(3\sqrt{x+5}=6\)

\(\sqrt{x+5}=2\)

\(\left\{{}\begin{matrix}2\ge0\left(ld\right)\\x+5=4\end{matrix}\right.\)

\(x=-1\)

a: \(A=\sqrt{4x+20}-2\sqrt{x+5}+\sqrt{9x+45}\)

\(=2\sqrt{x+5}-2\sqrt{x+5}+3\sqrt{x+5}\)

\(=3\sqrt{x+5}\)

b: A=6

=>\(3\sqrt{x+5}=6\)

=>\(\sqrt{x+5}=2\)

=>x+5=4

=>x=-1

\(a,\Leftrightarrow x-1=4\Leftrightarrow x=5\\ b,\Leftrightarrow\left\{{}\begin{matrix}x\ge\dfrac{3}{4}\\3x+1=4x-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x\ge\dfrac{3}{4}\\x=4\left(tm\right)\end{matrix}\right.\Leftrightarrow x=4\\ c,ĐK:x\ge-5\\ PT\Leftrightarrow2\sqrt{x+5}-3\sqrt{x+5}+4\sqrt{x+5}=6\\ \Leftrightarrow3\sqrt{x+5}=6\\ \Leftrightarrow\sqrt{x+5}=3\\ \Leftrightarrow x+5=9\\ \Leftrightarrow x=4\left(tm\right)\)

\(d,\Leftrightarrow\sqrt{\left(x-2\right)^2}=\sqrt{\left(\sqrt{5}+1\right)^2}\\ \Leftrightarrow\left|x-2\right|=\sqrt{5}+1\\ \Leftrightarrow\left[{}\begin{matrix}x-2=\sqrt{5}+1\\2-x=\sqrt{5}+1\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\sqrt{5}+3\\x=1-\sqrt{5}\end{matrix}\right.\)

a: ĐKXĐ: x∉{2;-2;3}

\(A=\left(\frac{4x}{x+2}-\frac{x^3-8}{x^3+8}\cdot\frac{4x^2-8x+16}{x^2-4}\right):\frac{16}{x^2-x-6}\)

\(=\left(\frac{4x}{x+2}-\frac{\left(x-2\right)\left(x^2+2x+4\right)}{\left(x+2\right)\left(x^2-2x+4\right)}\cdot\frac{4\left(x^2-2x+4\right)}{\left(x-2\right)\left(x+2\right)}\right):\frac{16}{\left(x-3\right)\left(x+2\right)}\)

\(=\left(\frac{4x}{x+2}-\frac{4\left(x^2+2x+4\right)}{\left(x+2\right)^2}\right)\cdot\frac{\left(x-3\right)\cdot\left(x+2\right)}{16}\)

\(=\frac{4x\left(x+2\right)-4\left(x^2+2x+4\right)}{\left(x+2\right)^2}\cdot\frac{\left(x-3\right)\left(x+2\right)}{16}=\frac{4x^2+8x-4x^2-8x-16}{\left(x+2\right)}\cdot\frac{x-3}{16}\)

\(=\frac{-16\left(x-3\right)}{16\left(x+2\right)}=\frac{-x+3}{x+2}\)

b: A<0

=>\(\frac{-x+3}{x+2}<0\)

=>\(\frac{x-3}{x+2}>0\)

=>x>3 hoặc x<-2

c: A>=5

=>\(\frac{-x+3}{x+2}-5\ge0\)

=>\(\frac{-x+3-5x-10}{x+2}\ge0\)

=>\(\frac{-6x-7}{x+2}\ge0\)

=>\(\frac{6x+7}{x+2}\le0\)

=>-2<x<=-7/6

b: \(B=\dfrac{x^2-3x+2x^2+6x-3x^2-9}{x^2-9}=\dfrac{3x-9}{\left(x-3\right)\left(x+3\right)}=\dfrac{3}{x+3}\)

$\textbf{a)}$

Ta có $x^3+5x^2-9x-45=0$

$\Leftrightarrow x^2(x+5)-9(x+5)=0$

$\Leftrightarrow(x+5)(x^2-9)=0.$

Vì $x\ne-5$ nên $x^2-9=0$

$\Rightarrow x=\pm3.$

Nhưng theo điều kiện xác định $x\ne\pm3.$

Vậy phương trình không có giá trị nào thỏa mãn điều kiện xác định nên $A$ không xác định.