Question 1117770
<font face="Times New Roman" size="+2">


You can do this one in your head in a lot less time than it takes to explain it.


The directrix is a horizontal line, so a parabola with vertex at the origin will have the form *[tex \Large y\ =\ ax^2].  So you can immediately exclude answers b. and d.  If the value *[tex \Large a] is positive, the parabola is concave UP.  If negative, concave DOWN.


Since the vertex is at the origin, and the directrix is at -2.5, the directrix is BELOW the vertex meaning that the parabola is concave up.  Consequently, you can now eliminate another answer leaving you with the correct one.


John
*[tex \LARGE e^{i\pi}\ +\ 1\ =\ 0]
My calculator said it, I believe it, that settles it
<img src="http://c0rk.blogs.com/gr0undzer0/darwin-fish.jpg">
*[tex \Large \ \
*[tex \LARGE \ \ \ \ \ \ \ \ \ \  

</font>