Question

resolva no grau no conjunto IR a) x² - x - 20 = 0

Answer 1

$x^{2} -x-20=0\\\\ a=1;b=-1;c=-20\\\\\\ \Delta=b^-4ac\to \Delta=(-1)^2-4.1.(-20)\to \Delta=1+80\to \boxed{\Delta=81}\\\\ x' \neq x''\\ \\\\ x= \dfrac{-b\pm \sqrt{\Delta} }{2a} \to x= \dfrac{-(-1)\pm \sqrt{81} }{2.1} \to x= \dfrac{1\pm9 }{2} \to \\\\\\x'= \dfrac{1+9 }{2} \to x'= \dfrac{10 }{2} \to x'=5\\\\\\ x''= \dfrac{1-9 }{2} \to x''= \dfrac{-8 }{2} \to x''=-4\\\\ \\\\\\\boxed{S=\{-4;5\}}$