Calculating a parabola: What am I doing wrong? [closed]
- by Nils
I was following this thread and copied the code in my project. Playing around with it turns out that it seems not to be very precise.
Recall the formula: y = ax^2 + bx +c
Since the first given point I have is at x1 = 0, we already have c=y1 . We just need to find a and b. Using:
y2 = ax2^2 + bx2 +c
y3 = ax3^2 + bx3 +c
Solving the equations for b yields:
b = y/x - ax - cx
Now setting both equations equal to each other so b falls out
y2/x2 - ax2 - cx2 = y3/x3 - ax3 - cx3
Now solving for a gives me:
a = ( x3*(y2 - c) + x2*(y3 - c) ) / ( x2*x3*(x2 - x3) )
(is that correct?!)
And then using again b = y2/x2 - ax2 - cx2 to find b. However so far I haven't found the correct a and b coeffs. What am I doing wrong?
