- AThe triangle PFQ is a right-angled triangle
- BThe triangle QPQ' is a right-angled triangle
- CThe distance between P and F is 5
- DF lies on the line joining Q and Q'
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Correct answer: A, B, D
Step 1: Analyze the Parabola and Given Points
The equation of the parabola E is given as .
This is a standard parabola of the form .
Comparing the two equations, we get 4a = 8, which implies a = 2.
- The focus F of the parabola is at
(a, 0), soF = (2, 0). - The vertex is at the origin
(0, 0). - The equation of the directrix is
x = -a, which isx = -2.
The given point is P = (-2, 4).
Step 2: Determine the Location of Point P
By observing the coordinates of point P (-2, 4), we can see that its x-coordinate is -2. This means that point P lies on the directrix of the parabola, x = -2.
This observation is key, as several properties of a parabola are related to its directrix.
Step 3: Evaluate each statement
A: The triangle PFQ is a right-angled triangle
There is a standard property of parabolas: The portion of a tangent between the point of contact and the directrix subtends a right angle at the focus.
In this case, PQ is a tangent to the parabola, where Q is the point of contact and P is a point on the directrix. According to this property, the line segment PQ must subtend a right angle at the focus F. This means that the angle ∠PFQ = 90°.
Therefore, the triangle PFQ is a right-angled triangle (right-angled at F).
Alternatively, using slopes:
Let the coordinates of Q be . The equation of the tangent at Q is , which is . Since this tangent passes through P(-2, 4), we have , which simplifies to . Let be the parameter for point Q.
Slope of PF: .
Slope of FQ: .
For ∠PFQ = 90°, we must have .
.
This is the same condition that must satisfy for the tangent to pass through P. Thus, the condition is met. Statement A is TRUE.
B: The triangle QPQ' is a right-angled triangle
Another standard property of parabolas is that tangents drawn from any point on the directrix are perpendicular to each other.
Since P (-2, 4) lies on the directrix, the two tangents PQ and PQ' must be perpendicular. This means the angle ∠QPQ' = 90°.
Therefore, the triangle QPQ' is a right-angled triangle (right-angled at P).
Alternatively, using parameters:
From the quadratic , let the roots be and , corresponding to points Q and Q'. The slope of the tangent at is 1/t. So the slopes of tangents PQ and PQ' are and .
The product of the slopes is . From Vieta's formulas for the quadratic, the product of roots . So, . The tangents are perpendicular. Statement B is TRUE.
C: The distance between P and F is 5√2
The coordinates are P (-2, 4) and F (2, 0). We use the distance formula:
.
The statement says the distance is , which is incorrect. Statement C is FALSE.
D: F lies on the line joining Q and Q'
A third standard property of parabolas is that the chord of contact of tangents drawn from any point on the directrix is a focal chord (i.e., it passes through the focus).
The line joining Q and Q' is the chord of contact for tangents drawn from point P. Since P lies on the directrix, the chord of contact QQ' must pass through the focus F. Therefore, F lies on the line joining Q and Q'.
Alternatively, using chord of contact equation:
The equation of the chord of contact from an external point to the parabola is .
For P (-2, 4) and a=2, the equation of QQ' is:
y(4) = 2(2)(x + (-2))
4y = 4(x - 2)
y = x - 2.
To check if the focus F (2, 0) lies on this line, substitute its coordinates into the equation:
0 = 2 - 2, which is 0 = 0. The condition is satisfied.
Statement D is TRUE.
Conclusion
Based on the analysis, statements A, B, and D are true, while statement C is false.
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