Question #224002

 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.


1
Expert's answer
2021-08-12T17:53:24-0400
x2y+4a=0x − 2y + 4a = 0

y2=4axy^2= 4ax

Then


y2=4a(2y4a)y^2=4a(2y-4a)

y28ay+16a2=0y^2-8ay+16a^2=0

(y4a)2=0(y-4a)^2=0

y=4ay=4a

Since the equation y2=4a(2y4a)y^2=4a(2y-4a) has the only root, then the line x2y+4a=0x − 2y + 4a = 0 touches the parabola y2=4ax.y^2=4ax.


y=4a,x=(4a)2/4a=4ay=4a, x=(4a)^2/4a=4a

The point of contact P(4a,4a).P(4a, 4a).


Suppose that QQ has coordinates (aq2,2aq)(aq^2, 2aq) and RR has coordinates (ar2,2ar).(ar^2, 2ar).

The line x2y+2a=0x − 2y + 2a = 0 meets the parabola in Q,RQ, R


aq24aq+2a=0aq^2-4aq+2a=0

q24q+2=0,a0q^2-4q+2=0, a\not=0

q=2±2q=2\pm\sqrt{2}


ar24ar+2a=0ar^2-4ar+2a=0

r24r+2=0,a0r^2-4r+2=0, a\not=0

r=2±2r=2\pm\sqrt{2}




(22)2=642(2-\sqrt{2})^2=6-4\sqrt{2}

(2+2)2=6+42(2+\sqrt{2})^2=6+4\sqrt{2}

Suppose that QQ has coordinates (a(642),a(422))(a(6-4\sqrt{2}), a(4-2\sqrt{2})) and RR has coordinates (a(6+42),a(4+22)).(a(6+4\sqrt{2}), a(4+2\sqrt{2})).

Since MM is the mid-point of QR,QR, MM has coordinates (6a,4a).( 6a, 4a).

Then the equation of PMPM is y=4a.y=4a.

Hence PMPM is parallel to the axis of x.x.


Find the equation of the normal at PP to the parabola


slope=112=2slope=\dfrac{-1}{\dfrac{1}{2}}=-2


y=2x+by=-2x+b

4a=2(4a)+b=>b=12a4a=-2(4a)+b=>b=12a


y=2x+12ay=-2x+12a

The lines y=0,x=6a,y=0, x=6a, and y=2x+12ay=-2x+12a meet at the point (6a,0).(6a, 0).



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