Question

8) A função f(x) = - x² - 6x - 9 corta o eixo x em: a) x’ = 1 e x” = 1 b) x’ = -3 e x” = -3 c) x’ = 1 e x” = -3 d) x’ = -1 e x” = 3

Answer 1

$f(x)=-x^2-6x-9\Longrightarrow a=-1,\ b=-6,\ c=-9.$

$\mathrm{Ra\acute{\i}zes\ de}\ f(x)\Longrightarrow\ f(x)=0\Longrightarrow x=\dfrac{-b\pm\sqrt{b^2-4ac}}{2a}$

$\Longrightarrow x=\dfrac{-(-6)\pm\sqrt{(-6)^2-4(-1)(-9)}}{2(-1)}=\dfrac{6\pm0}{-2}\ \therefore\ \boxed{x=-3}$

$\Longrightarrow x'=-3\ \text{e}\ x''=-3\Longrightarrow \mathrm{Letra\ B.}$