[tex]\displaystyle\bf\\20e)\\\\-7x^2+28=0\\\\-7x^2=-28\\\\x^2=\frac{-28}{-7}\\\\x^2=4\\\\x_{12}=\pm\sqrt{4}\\\\x_{12}=\pm2\\\\\boxed{\bf x_1=2~~si~~x_2=-2}\\\\\\20f)\\\\5x^2-125=0\\\\5x^2=125\\\\x^2=\frac{125}{5}\\\\x^2=25\\\\x_{12}=\pm\sqrt{25}\\\\x_{12}=\pm5\\\\\boxed{\bf x_1=5~~si~~x_2=-5}[/tex]
