The general equation of a circle in the plane is?
A
x² + y² + 2gx + 2fy + c = 0
B
(x - h)² + (y - k)² = r²
C
ax² + by² + 2gx + 2fy + c = 0 with a=b
D
Both 1 and 3
Analysis & Theory
The general form is x² + y² + 2gx + 2fy + c = 0, or equivalently ax² + by² + ... with a=b=1.
In the general equation x² + y² + 2gx + 2fy + c = 0, the center of the circle is?
A
(g, f)
B
(-g, -f)
C
(c, g)
D
(-c, -f)
Analysis & Theory
Comparing with (x - h)² + (y - k)² = r², we get center = (-g, -f).
In the general equation x² + y² + 2gx + 2fy + c = 0, the radius of the circle is?
A
√(g² + f² + c)
B
√(g² + f² - c)
C
√(g² - f² - c)
D
√(g² + f²)
Analysis & Theory
Radius = √(g² + f² - c).
The equation x² + y² - 4x - 6y + 9 = 0 represents a circle with center?
A
(2, 3)
B
(–2, –3)
C
(2, –3)
D
(–2, 3)
Analysis & Theory
Here 2g = -4 ⇒ g = -2, 2f = -6 ⇒ f = -3 ⇒ center = (-g, -f) = (2, 3).
The radius of the circle x² + y² - 4x - 6y + 9 = 0 is?
A
2
B
3
C
4
D
5
Analysis & Theory
Radius = √(g² + f² - c) = √((-2)² + (-3)² - 9) = √(13 - 9) = √4 = 2. Correction: Actually c = 9, so radius = √(13 - 9) = 2.
The equation x² + y² - 2x + 4y - 11 = 0 has radius?
A
2
B
3
C
4
D
5
Analysis & Theory
g = -1, f = 2, c = -11. Radius = √(1 + 4 - (-11)) = √16 = 4.
Which of the following represents a circle of zero radius (point circle)?
A
x² + y² = 0
B
x² + y² + 2gx + 2fy + g² + f² = 0
C
(x - h)² + (y - k)² = 0
D
All of the above
Analysis & Theory
All these equations reduce to a single point circle.
For x² + y² + 6x - 8y + 9 = 0, the center is?
A
(–3, 4)
B
(3, –4)
C
(–6, 8)
D
(6, –8)
Analysis & Theory
g = 3, f = -4 ⇒ center = (–g, –f) = (–3, 4).
For x² + y² + 6x - 8y + 9 = 0, the radius is?
A
√16
B
4
C
√7
D
√25
Analysis & Theory
Radius = √(g² + f² - c) = √(9 + 16 - 9) = √16 = 4.
The equation x² + y² + 2gx + 2fy + c = 0 does not represent a real circle when?
A
g² + f² - c < 0
B
g² + f² - c > 0
C
g² + f² - c = 0
D
None of these
Analysis & Theory
If g² + f² - c < 0, the radius becomes imaginary, so no real circle exists.