`#Sad`

`3)`

`a)`

`(x+2)(x-2)-(x-3)^2 = 5`

`⇔ x^2-4-x^2+6x-9-5 = 0`

`⇔ 6x-18 = 0`

`⇔ 6x = 18`

`⇔ x = 3`

`\text{Vậy}` `x=3`

`b)`

`x^2-4x+3 = 0`

`⇔ x^2-3x-x+3 = 0`

`⇔ (x^2-3x)-(x-3) = 0`

`⇔ x(x-3)-(x-3) = 0`

`⇔ (x-1)(x-3) = 0`

`⇔` \(\left[ \begin{array}{l}x-1=0\\x-3=0\end{array} \right.\) 

`⇔` \(\left[ \begin{array}{l}x=1\\x=3\end{array} \right.\) 

`\text{Vậy}` `x=1` `\text{hoặc}` `x=3`

`4)`

`7^(2021)-7^(2000)`

`= 7^(2000). (7-1)`

`= 7^(2000). 6`

`\text{Vì}` `6` `\vdots` `3`

`⇒ 7^(2000). 6` `\vdots` `3`

`⇒` `7^(2021)-7^(2000)` `\vdots` `3`

Giải thích các bước giải:

`3)`

`a)`

`(x + 2)(x - 2) - (x - 3)^2 = 5`

`⇔ x^2 - 4 - (x^2 - 6x + 9) = 5`

`⇔ x^2 - x^2 + 6x - 4 - 9 = 5`

`⇔ 6x = 18`

`⇔ x = 3`

`Vậy  x = 3`

`b)`

`x^2 - 4x + 3 = 0`

`⇔ x^2 - x - 3x + 3 = 0`

`⇔ x(x - 1) - 3(x - 1) = 0`

`⇔ (x - 3)(x - 1) = 0`

`⇒` \(\left[ \begin{array}{l}x=3\\x=1\end{array} \right.\) 

`Vậy  S = {3 ; 1}`

`4)`

`7^2021 - 7^2020`

`= 7^2020(7 - 1)`

`= 7^2020 . 6 \vdots 3`

`⇒ 7^2021 - 7^2020 \vdots 3`