tim x biet a,25x^2 -9=0 b,(x+4)^2 -(x+1)*(x-6)=16 c,(2x-1)^2 +(x+3)^2 -5*(x+7)*(x-7)=0

$a)25x² -9=0$

$(=)(5x-3)(5x+3)=0$

$(=)$\(\left[ \begin{array}{l}5x-3=0\\5x+3=0\end{array} \right.\) $(=)$\(\left[ \begin{array}{l}5x=3\\5x=-3\end{array} \right.\)$(=)$ \(\left[ \begin{array}{l}x=\frac{3}{5}\\x=-\frac{3}{5}\end{array} \right.\)

$S={{\frac{3}{5};\frac{-3}{5}}}$

$b)(x+4)²-(x+1)(x-6)=16$

$(=)x²+8x+16-(x²-6x+x-6)=16$

$(=)x²+8x+16-x²+6x-x+6=16$

$(=)13x+22=16$

$(=)13x=16-22$

$(=)13x=-6$

$(=)x=\frac{-6}{13}$

$S$={$\frac{-6}{13}$}

$c)(2x-1)² +(x+3)² -5(x+7)(x-7)=0$

$(=)4x²-4x+1+x²+6x+9-5(x²-49)=0$

$(=)4x²-4x+1+x²+6x+9-5x²+245=0$

$(=)2x+255=0$

$(=)2x=-255$

$(=)x=-\frac{255}{2}$ 

$S={-\frac{255}{2}}$