Đáp án:

 $2)
a) C=\{2x+1|x\in \mathbb{N}, 0\leq x\leq 9\}\\ $ $b)
 D=\{3x+1| x\in \mathbb{N},0\leq x\leq 10 \}\\
3)
e)A=\{-4;6\}\\
f)
A=\{1\}\\
g)  A=\{\pm 2\}\\
h)
 A=\{-1;1\}\\
m) A=\{4\}\\
n)  x=\{1\}\\
p) 
A=\{1\}\\
q)
A=\{1\}\\
o)
 A=\{9\}$

Giải thích các bước giải:

 $2)
a) C=\{1;3;5;...;19\}\Rightarrow C=\{2x+1|x\in \mathbb{N}, 0\leq x\leq 9\}\\
b)
D=\{1;4;7;10;...;31\}\Rightarrow D=\{3x+1| x\in \mathbb{N},0\leq x\leq 10 \}\\
3)
e) (x-1)^2=25\Leftrightarrow  x-1=\pm 5\\
\Leftrightarrow {\left[\begin{aligned}x-1=5\\x-1=-5\end{aligned}\right.}\\
\Leftrightarrow  {\left[\begin{aligned}x=6\\x=-4\end{aligned}\right.}\\
A=\{-4;6\}\\
f)
(3-1)^2-4x=1^2+2^2-5\\
\Leftrightarrow 2^2-4x=1+4-5\\
\Leftrightarrow 4-4x=0\\
\Leftrightarrow 4x=4\\
\Leftrightarrow x=1\\
A=\{1\}\\
g) (3-1)^2.x^2+1^{100}=17\\
\Leftrightarrow 2^2.x^2+1=17\\
\Leftrightarrow 4x^2=17-1=16\\
\Leftrightarrow x^2=4\\
\Leftrightarrow x=\pm 2\\
\Rightarrow A=\{\pm 2\}\\
h)
4^2-x^2=15\\
\Leftrightarrow x^2=4^2-15\\
\Leftrightarrow x^2=16-15=1\\
\Leftrightarrow x=\pm 1\\
\Rightarrow A=\{-1;1\}\\
m) 4.3^x=2^2.81\\
\Leftrightarrow 3^x=\dfrac{4.81}{4}=81\\
\Leftrightarrow 3^x=3^4\\
\Leftrightarrow x=4\\
\Rightarrow A=\{4\}\\
n) (3^{100}:3^{98}).3^x=27\\
\Leftrightarrow 3^2.3^x=27\\
\Leftrightarrow 3^x=3\\
\Leftrightarrow x=1\\
\Rightarrow x=\{1\}\\
p) (4.2^{10}+28.2^{10}).x=2^{19}:2^{3}\\
\Leftrightarrow 2^{10}(4+28)x=2^{16}\\
\Leftrightarrow 32x=2^6\\
\Leftrightarrow 2^6x=2^6\\
\Leftrightarrow x=1\\
A=\{1\}\\
q)
\left ( \dfrac{2}{9}^2+\dfrac{5}{9} \right )x-\dfrac{1}{2}=\dfrac{1}{2}\\
\Leftrightarrow \dfrac{9}{9}x=\dfrac{1}{2}+\dfrac{1}{2}\\
\Leftrightarrow x=1\\
\Rightarrow A=\{1\}\\
o)
\left ( \dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4} \right ).\dfrac{4}{3}x=3^2\\
\Leftrightarrow \left ( \dfrac{1}{1}-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{4} \right ).\dfrac{4}{3}x=3^2\\
\Leftrightarrow \left ( 1-\dfrac{1}{4} \right ).\dfrac{4}{3}x=9\\
\Leftrightarrow \dfrac{3}{4}.\dfrac{4}{3}x=9\\
\Leftrightarrow x=9\\
\Rightarrow A=\{9\}$