SOLUTION: Find an equation of a parabola with a vertex at the origin and directrix y=-2.5
A) {{{y=-(1/10)x^2}}}
B) {{{x=(1/10)y^2}}}
C) {{{y=(1/10)x^2}}}
D) {{{x=-(1/10)y^2}}}
Algebra.Com
Question 1117770: Find an equation of a parabola with a vertex at the origin and directrix y=-2.5
A)
B)
C)
D)
Answer by solver91311(24713) (Show Source): You can put this solution on YOUR website!
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 
. So you can immediately exclude answers b. and d. If the value 
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
My calculator said it, I believe it, that settles it
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