Two parabolas have the same focus (4,3) and their directrices are the x-axis and the y-axis, respectively. If these parabolas intersects at the points A and B , then (AB)2 is equal to
(A)192✓ Correct
(B)384
(C)96
(D)392
Answer: (A)
Solution
For the parabola with focus (4,3) and directrix the x-axis, a point (x,y) satisfies
(x−4)2+(y−3)2=y2⟹x2−8x−6y+25=0.(1)
For the one with directrix the y-axis,
(x−4)2+(y−3)2=x2⟹y2−8x−6y+25=0.(2)
Subtracting, x2−y2=0, so x=y or x=−y.
Substituting x=−y into (1) gives x2−2x+25=0, whose discriminant is negative, so there is no real solution there.
Substituting x=y into (1) gives
x2−14x+25=0,
with roots x1,2=7±26 (so x1+x2=14, x1x2=25). The two intersection points are (x1,x1) and (x2,x2), so