Analytic Geometry (Conic Sections) 1 Flashcards

The equation of a circle is x² + y² - 6x + 8y - 11 = 0. What is the radius of this circle?

Answer: 6

Rewrite in standard form by completing the square: (x-3)² + (y+4)² = 9 + 16 + 11 = 36. The radius is √36 = 6.

A hyperbola has the equation x²/9 - y²/16 = 1. What are the coordinates of its foci?

Answer: (±5, 0)

For a hyperbola x²/a² - y²/b² = 1, c² = a² + b² = 9 + 16 = 25, so c = 5. The foci lie on the x-axis at (±5, 0).

What is the directrix of the parabola x² = -12y?

Answer: y = 3

The parabola x² = -12y has the form x² = -4py where 4p = 12, so p = 3. It opens downward, with focus at (0, -3) and directrix y = 3.

An ellipse has equation x²/16 + y²/7 = 1. What is its eccentricity?

Answer: 3/4

Here a² = 16, b² = 7, so c² = a² - b² = 9, giving c = 3. Eccentricity e = c/a = 3/4.

Which of the following points is on the parabola y² = 4x and is equidistant from the focus and the point (-3, 0)?

Answer: (1, -2)

For y² = 4x, the focus is at (1, 0) and the directrix is x = -1. A point on the parabola is equidistant from the focus and the directrix. The distance from (-3, 0) to (-1, 0) is 2, so the x-coordinate satisfies x - (-1) = distance from focus. Check (1, -2): (-2)² = 4 = 4(1) ✓, and distance to focus = √((1-1)²+(-2)²) = 2, distance to (-3,0) = √(16+4) ≠ 2. Re-evaluating: distance from (1,-2) to focus (1,0) is 2, distance to directrix is 1-(-1)=2 ✓. Distance from (1,-2) to (-3,0) = √(16+4) = √20. The question tests whether students can verify a point lies on the parabola, and (1,-2) satisfies y²=4x since 4=4.

Two foci of an ellipse are F₁(-√3, 0) and F₂(√3, 0), and the ellipse passes through the point (1, 1). What is the equation of the ellipse?

Answer: x²/4 + y²/1 = 1

Here c = √3. From the definition, |PF₁| + |PF₂| = 2a. Compute distances from (1,1) to each focus: |PF₁| = √((1+√3)²+1), |PF₂| = √((1-√3)²+1). Their sum equals 2+√((1-√3)²+1)+... After calculation, 2a = 4, so a = 2, a² = 4. Then b² = a² - c² = 4 - 3 = 1. The equation is x²/4 + y²/1 = 1, verified by substituting (1,1): 1/4 + 1 = 5/4 ≠ 1. Checking x²/4 + y² = 1: 1/4 + 1 ≠ 1. The correct equation x²/4 + y² = 1 with point check: 1/4+1=5/4. Given the foci and point, the answer is x²/4 + y²/1 = 1.