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`\color{white}{a,}`

`49.(x - 4)^2 - 169 = 0`

`<=> [7.(x - 4)]^2 - 13^2 = 0`

`<=> [7.(x - 4) - 13].[7.(x - 4) + 13] = 0`

`<=> (7x - 28 - 13).(7x - 28 + 13) = 0`

`<=> (7x - 41).(7x - 15) = 0`

`<=>` \(\left[ \begin{array}{l}7x - 41 = 0\\7x - 15 = 0\end{array} \right.\)

`<=>` \(\left[ \begin{array}{l}7x = 41\\7x = 15\end{array} \right.\)

`<=>` \(\left[ \begin{array}{l}x = \dfrac{41}7\\x = \dfrac{15}7\end{array} \right.\)

Vậy `x in {15/7 ; 41/7}`

$\\$

`\color{white}{b,}`

`(x + 2)^3 + (x -3)^3 = 0`

`<=> x^3 + 6x^2 + 12x +  8 + x^3 - 9x^2 + 27x - 27 = 0`

`<=> 2x^3−3x^2+39x−19 = 0`

`<=> (2x^3 - x^2) - (2x^2 - x) + (38x - 19) = 0`

`<=> x^2.(2x - 1) - x.(2x - 1) + 19.(2x - 1) = 0`

`<=> (2x - 1).(x^2 - x + 19) = 0`

`<=> 2x-  1 = 0` `(x^2 - x + 19 > 0)`

`<=> 2x = 1`

`<=> x = 1/2`

Vậy `x = 1/2`

$\\$

`\color{white}{c,}`

`(x^2 - 5)^2 - 121 = 0`

`<=> (x^2 - 5)^2 - 11^2 = 0`

`<=> (x^2 - 5 - 11).(x^2 - 5 + 11) = 0`

`<=> (x^2 - 16).(x^2 + 6) = 0`

`<=> x^2 - 16 = 0` `(x^2 + 6 > 0)`

`<=> x^2 = 16`

`<=> x = +- 4`

Vậy `x = - 4`

$@Ri$