Find the x,y intercepts?

[justan4cat]

The question is: Find the x and y intercepts.

The equation is: f(x)=-x^2-4x-18

When x=0, then y=-18

For the x intercept, I tried to solve for x:
-x^2-4x=18
divide both sides by -1 and get
x^2+4x=-18
complete the square add 4 to both sides
x^2+4x+4=-14
(x+2)^2=-14
x+2=+/- sqrt -14
x=+/- sqrt (-14)-2

I don't even come close to the the possibilities of answers provided! What am I doing wrong? However, the choices for the multiple choice answers are: (please excuse the errors in the code, I don't have time to play with the MathType addon for Word.

\(\displaystyle \begin{gathered}
\left( {\frac{{4 \pm \sqrt {88} }}{2},0} \right),\left( {0,18} \right) \hfill \\
\left( {\frac{{4 \pm \sqrt {88} }}{2},0} \right),\left( {0, - 18} \right) \hfill \\
\end{gathered}\)
No x-intercepts, (0,18)
No x-intercepts, (0,-18)

 

[masters]

The question is: Find the x and y intercepts.

The equation is: f(x)=-x^2-4x-18

When x=0, then y=-18

\(\displaystyle \begin{gathered}
\left( {\frac{{4 \pm \sqrt {88} }}{2},0} \right),\left( {0,18} \right) \hfill \\
\left( {\frac{{4 \pm \sqrt {88} }}{2},0} \right),\left( {0, - 18} \right) \hfill \\
\end{gathered}\)
No x-intercepts, (0,18)
No x-intercepts, (0,-18)

Hi justan4cat,

\(\displaystyle f(x)=-x^2-4x-18\) has no x-intercepts.

Use the quadratic formula (or complete the square as you did) to determine that the zeros are imaginary.

\(\displaystyle x=\frac{-b \pm \sqrt{b^2-4ac}}{2a}\)

\(\displaystyle x=\pm i \sqrt{14}-2\)

Your y-intercept of (0, -18) is correct.

 

[BigGlenntheHeavy]

\(\displaystyle Graph \ of \ f(x) \ = \ -x^2-4x-18, \ see \ below, \ need \ anymore \ be \ said?\)

\(\displaystyle Old \ Chinese \ proverb: \ No \ tickee, \ no \ washee, \ oops, \ wrong \ one, \ I \ meant \ a \ picture \ is \ worth\)

\(\displaystyle a \ thousand \ words.\)

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