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AP EAMCET · Maths · Circle

Let \(\theta\) be the angle between the circles \(S \equiv x^2+y^2+2 x-2 y+c=0\) and \(S^1 \equiv x^2+y^2-6 x-8 y+9=0\). If \(c\) is an integer and \(\cos \theta=\frac{5}{16}\) then the radius of the circle \(\mathrm{S}=0\) is

  1. A 2
  2. B 4
  3. C 3
  4. D 1
Verified Solution

Answer & Solution

Correct Answer

(A) 2

Step-by-step Solution

Detailed explanation

\(C_1=(-1,1)\), \(r_1=\sqrt{1^2+(-1)^2-c}=\sqrt{2-c}\) \(C_2=(3,4)\), \(r_2=\sqrt{(-3)^2+(-4)^2-9}=\sqrt{16}=4\) \(d=\sqrt{(3-(-1))^2+(4-1)^2}=\sqrt{4^2+3^2}=\sqrt{25}=5\) \(\cos \theta = \frac{d^2 - r_1^2 - r_2^2}{2 r_1 r_2}\)…