Source: Extra Practice

The length of the common chord of the circles x2+y2+2x+3y+1=0x^2 + y^2 + 2x + 3y + 1 = 0 and x2+y2+4x+3y+2=0x^2 + y^2 + 4x + 3y + 2 = 0 is:

Options

Option A is correct

222\sqrt{2}

Option B

2\sqrt{2}

Option C

232\sqrt{3}

Option D

3\sqrt{3}

Explanation

Let the two circles be S1:x2+y2+2x+3y+1=0S_1: x^2 + y^2 + 2x + 3y + 1 = 0 and S2:x2+y2+4x+3y+2=0S_2: x^2 + y^2 + 4x + 3y + 2 = 0. The equation of the common chord (radical axis) is S1−S2=0S_1 - S_2 = 0. (x2+y2+2x+3y+1)−(x2+y2+4x+3y+2)=0(x^2 + y^2 + 2x + 3y + 1) - (x^2 + y^2 + 4x + 3y + 2) = 0 −2x−1=0⇒2x+1=0⇒x=−12-2x - 1 = 0 \Rightarrow 2x + 1 = 0 \Rightarrow x = -\frac{1}{2}.

For circle S1S_1, the center C1=(−1,−32)C_1 = (-1, -\frac{3}{2}) and radius r1=(−1)2+(−32)2−1=1+94−1=94=32r_1 = \sqrt{(-1)^2 + (-\frac{3}{2})^2 - 1} = \sqrt{1 + \frac{9}{4} - 1} = \sqrt{\frac{9}{4}} = \frac{3}{2}. The distance dd from the center C1C_1 to the common chord x=−12x = -\frac{1}{2} is d=∣−1−(−12)∣=∣−1+12∣=∣−12∣=12d = |-1 - (-\frac{1}{2})| = |-1 + \frac{1}{2}| = |-\frac{1}{2}| = \frac{1}{2}.

The length of half the common chord, say L/2L/2, can be found using the Pythagorean theorem: (L/2)2=r12−d2(L/2)^2 = r_1^2 - d^2. L/2=r12−d2=(32)2−(12)2=94−14=84=2L/2 = \sqrt{r_1^2 - d^2} = \sqrt{(\frac{3}{2})^2 - (\frac{1}{2})^2} = \sqrt{\frac{9}{4} - \frac{1}{4}} = \sqrt{\frac{8}{4}} = \sqrt{2}. The total length of the common chord is 2×(L/2)=222 \times (L/2) = 2\sqrt{2}.

Options B, C, and D provide incorrect lengths, as they do not result from the correct application of the distance and Pythagorean formulas.