`(5x-4)^2 - 49x^2 = 0`

`(5x-4)^2 - (7x)^2 = 0`

`(5x-4-7x)(5x-4+7x) = 0`

`(-2x-4)(12x-4)=0`

$\left[\begin{matrix} -2x-4=0\\ 12x-4=0\end{matrix}\right.$

$\left[\begin{matrix} x=-2\\ x=\dfrac{1}{3}\end{matrix}\right.$

Vậy `x in { -2 ; 1/3 }`

`---------------`

`16.(x-1)^2 - 25 = 0`

`[4.(x-1)]^2 - 5^2 = 0`

`[4(x-1)-5].[4(x-1)+5] = 0`

`(4x-4-5)(4x-4+5)=0`

`(4x-9=0)(4x+1)=0`

$\left[\begin{matrix} 4x-9=0\\ 4x+1=0\end{matrix}\right.$

$\left[\begin{matrix} x=\dfrac{9}{4}\\ x=-\dfrac{1}{4}\end{matrix}\right.$

Vậy `x in { 9/4 ; -1/4 }`

`---------------`

`\star` HĐT áp dụng `:`

`a^2 - b^2 = (a-b)(a+b)`