Name: Reb
Who is asking: Student
Level of the question: Secondary

Question: Hey
i know how to convert the general formula into specific ones(ie.; a circle's specific formula x2 + y2 = r2 can be derived from this, and then you draw you graph), but i have no idea how to go from a graph to the general formula (you know the HUUUGE one...)



If all you have is a graph then you need to be able to tell enough from the graph to write an equation. You mentioned a circle, so if you have the graph of a circle and you can tell that the centre is at (2, -3) and the radius is 5 then you know that the equation is

(x - 2)2 + (y + 3)2 = 52

Now expand and collect terms

(x - 2)2 + (y + 3)2 = 52
x2 - 4 x + 4 + y2 + 6 y + 9 = 25
x2 - 4 x + y2 + 6 y - 12 = 0

The difficult step of course is looking at the graph and being able to determine the centre and the radius.