The vertex of a quadratic function can be found by using the following expression:
[tex]x=\frac{-b}{2a}[/tex]
Where "a" is the number multiplying x² and b is the number multiplying x. For this function a = -2 and b = 5. Applying these on the problem we have:
[tex]x=\frac{-5}{2\cdot(-2)}=\frac{-5}{-4}=\frac{5}{4}=1.25[/tex]
To find the y coordinate of the vertex we need to use the value for x that we found above. We have:
[tex]\begin{gathered} f(x)=-2x^2+5x+11 \\ f(\frac{5}{4})=-2\cdot(\frac{5}{4})^2+5\cdot(\frac{5}{4})+11 \\ f(\frac{5}{4})=-2\frac{25}{16}+\frac{25}{4}+11 \\ f(\frac{5}{4})=\frac{-50}{16}+\frac{25}{4}+11 \\ f(\frac{5}{4})=-3.125+6.25+11=14.125 \end{gathered}[/tex]
The ordered pair for this function's vertex is (1.25, 14.125)
