
Dayv O. answered 06/08/21
Attentive Reliable Knowledgeable Math Tutor
if the simple x=Ay2 is examined for distance to focus from vertex (equal distance from vertex to directix),
at y0=1/2A,,,,x=1/4A =-5 in this case, must go minus five units x from vertex to reach focus.which is the distance to focus from vertex (called p in thius problem). Vertex is at (8,-2), focus is at (3,-2). Some people consider p to be a distance always positive, and then p=5.
What I am doing in the above is setting up a square. At y=1/2A, the horizontal line through it is obviously perpendicular to directix Distance to directix to curve is 2p. Becuase of the definition of parabola that equals the distance from the foucs to the point.drawn parallel to the x-axis. (we have squaresince y=1/2A and pependicular side is 2*(1/4A)). Now 2p=1/2A. I did this at the origin because moving a curve does not change distance measurements between points. Look at solving 2x=y for square where x=Ay2, or 2Ay2=y which is at y=1/2A
second question, method is to eliminate the 1 from the equation
This is a valid method because when x is large the values of y approach the assymptope. Remember+/-a=+/-b is same as a=+/-b. see y=+/-√[(x-a)2-1]+k is very much the same as y= +/-√[(x-a)2]+k= +/-(x-a)+k