giải phương trình: $\frac{1}{(x+29)^2}$ +$\frac{1}{(x+30)^2}$ =$\frac{13}{36}$

$\begin{array}{l} Đặt x + 29 = a\\  \Rightarrow \dfrac{1}{{{{\left( {x + 29} \right)}^2}}} + \dfrac{1}{{{{\left( {x + 30} \right)}^2}}} = \dfrac{{13}}{{36}}\\  \Leftrightarrow \dfrac{1}{{{a^2}}} + \dfrac{1}{{{{\left( {a + 1} \right)}^2}}} = \dfrac{{13}}{{36}}\\  \Leftrightarrow 36\left( {{a^2} + {a^2} + 2a + 1} \right) = 13{a^2}\left( {{a^2} + 2a + 1} \right)\\  \Leftrightarrow 13{a^4} + 26{a^3} – 59{a^2} – 72a – 36 = 0\\  \Leftrightarrow 13{a^4} – 26{a^3} + 52{a^3} – 104{a^2} + 45{a^2} – 90a + 18a – 36 = 0\\  \Leftrightarrow \left( {a – 2} \right)\left( {13{a^3} + 52{a^2} + 45a + 18} \right) = 0\\  \Leftrightarrow \left( {a – 2} \right)\left( {a + 3} \right)\left( {13{a^2} + 13a + 6} \right) = 0\\  \Leftrightarrow \left[ \begin{array}{l} a = 2\\ a =  – 3 \end{array} \right.\\  \Rightarrow \left[ \begin{array}{l} x + 29 = 2\\ x + 29 =  – 3 \end{array} \right. \Leftrightarrow \left[ \begin{array}{l} x =  – 27\\ x =  – 32 \end{array} \right.\\  \Rightarrow S = \left\{ { – 27 – 32} \right\} \end{array}$