Prove that the line x − 2y + 4a = 0 touches the parabola y2= 4ax, and find the coordinates of P, the point of contact. If the line x − 2y + 2a = 0 meets the parabola in Q, R, and M is the mid-point of QR, prove that PM is parallel to the axis of x, and that this axis and the line through M perpendicular to it meet on the normal at P to the parabola.
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Expert's answer
2021-08-12T17:53:24-0400
x−2y+4a=0
y2=4ax
Then
y2=4a(2y−4a)
y2−8ay+16a2=0
(y−4a)2=0
y=4a
Since the equation y2=4a(2y−4a) has the only root, then the line x−2y+4a=0 touches the parabola y2=4ax.
y=4a,x=(4a)2/4a=4a
The point of contact P(4a,4a).
Suppose that Q has coordinates (aq2,2aq) and R has coordinates (ar2,2ar).
The line x−2y+2a=0 meets the parabola in Q,R
aq2−4aq+2a=0
q2−4q+2=0,a=0
q=2±2
ar2−4ar+2a=0
r2−4r+2=0,a=0
r=2±2
(2−2)2=6−42
(2+2)2=6+42
Suppose that Q has coordinates (a(6−42),a(4−22)) and R has coordinates (a(6+42),a(4+22)).
Since M is the mid-point of QR,M has coordinates (6a,4a).
Then the equation of PM is y=4a.
Hence PM is parallel to the axis of x.
Find the equation of the normal at P to the parabola
slope=21−1=−2
y=−2x+b
4a=−2(4a)+b=>b=12a
y=−2x+12a
The lines y=0,x=6a, and y=−2x+12a meet at the point (6a,0).
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