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# Parabola equation

## Parabola equation

(OP)
Hi All,

I'm trying to figure out how to plot a parabola when I know the width and height of the parabola - I'm familiar with the standard parabola equation:

#### CODE

f(x)=y=ax2+bx+c
but I'm unclear as to what the variables other than x and y mean.

Any help would be appreciated.

Thanks
Todd

### RE: Parabola equation

i guess what you're saying is that you know two points on the parabola (since there is no end to a parabola (as a math function) and therefore no height or width).  using your terminology is guess the points you have are (-b/2, h) and (b/2,h)

i guess that you also know where the turning point is (0,0).

with three point you can substitute into the general parabola equation and solve the three equations for a, b, and c.  there are probably short-cuts, but pain is good.

### RE: Parabola equation

(OP)
Hi rb1957,

I use this for spinners on airplane propellers.  I realize there's probably a million ways spinners are designed, but right now (for the last several years anyway), I'm into parabolas for them - so essentially I know the base diameter - the max width for my purposes, and the length of the spinner - the max height for my purposes.  So these are the points I know, does that make sense?

### RE: Parabola equation

yeah, you know the width and the height ...

so you know three points (0,0), (-w/2,h), (w/2,h).

substitute for (x,y) and get three equations in a, b, and c.

solve ...

btw, c = 0

### RE: Parabola equation

(OP)
Got it - Thanks rb1957!

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