1.The points P(ap2 , 2ap) and Q(aq2 , 2aq) lie on the parabola y 2 = 4ax. Prove that if P Q is a focal chord then the tangents to the curve at P and Q intersect at right angles at a point on the directrix.
2. The tangents at the points P(ap2 , 2ap) and Q(aq2 , 2aq) on the parabola y 2 = 4ax intersect at the point R. Given that the tangent at P is perpendicular to the chord OQ, where O is the origin, find the equation of the locus of R as p varies.
3. The coordinates of the ends of a focal chord of the parabola y 2 = 4ax are (x1, y1) and (x2, y2). Show that x1x2 = a 2 and y1y2 = −4a 2 .
4. Prove that the line x − 2y + 4a = 0 touches the parabola y 2 = 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) Because (P,Q) is focal hord we have:
;
Let us find the intersection of Z(-a,u(p)) tangent at P with asymptota x=-a:
Similarly u(q)=a(q+p)
Thus u(p)=u(q) therefore tangents at P and Q intersecs at Z(-a,a(p+q));
k(P)=dy(p)/dx(P)= is the slope of tangent at P;
Similarly k(Q)= .
Therefore k(P) k(Q)=-1 and tangents at P and at Q are ortogonal in Z.
Part 1 is done.
2) Solution of part 2.
Let us find the equation of QO:
Its equation is given by Y=
From the part 1 Y=ap+X/p is the equation of tangent at P;
Because of OQ orthogonal to tangent at P we have
Therefore q=-2/p.
Point R(p) satisfies the system:
We solve this system by the method of determinants:
R(p)=(-2a,a(p-2/p)) is the parametric equation of locus, it is vertical line parallel to asymptota.
3) Solution of the third subproblem.
We have
is the equation of focal hord;
, the first statement is proved
, the second statement is proved
4) Solution of the fourth part:
Let P(ap2,2ap) be a point of contact of some tangent to parabola.
We saw in the first task that equation of any tangent is
For given line we have y=1/2∙x+2a
Therefore it is tangent line with p=2 and P(4a,4a)- the point of contact.
Let us find intersections Q,R of line x − 2y + 2a = 0 with parabola y 2 = 4ax.
x=2y-2a;
y2-8ay+8a2=0;
We see that yM=yP=>MP||OX
Let us prove that ZP, where Z(6a,0), P(4a,4a), is a normal to parabola.
Normal to the line x − 2y + 4a = 0 at P(4a,4a) has an equation
2x+y-12a=0. By substituting coordinates of Z(6a,0) in the last equation we
have 8a+4a-12a=0=>ZP is normal to parabola.
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