Question:

The equation of the circle passing through \((0,0)\) and cutting orthogonally the circles \[ x^2+y^2+6x-15=0 \] and \[ x^2+y^2-8y-10=0 \] is:

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Two circles \[ x^2+y^2+2g_1x+2f_1y+c_1=0 \] and \[ x^2+y^2+2g_2x+2f_2y+c_2=0 \] cut orthogonally if \[ 2g_1g_2+2f_1f_2=c_1+c_2. \]
Updated On: Jun 26, 2026
  • \(2(x^2+y^2)-10x+5y=0\)
  • \(2(x^2+y^2)+10x-5y=0\)
  • \(2(x^2-y^2)+10x+5y=0\)
  • \(2(x^2-y^2)-10x-5y=0\)
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The Correct Option is A

Solution and Explanation

Step 1: Assume the required circle.
Since the circle passes through the origin, its equation may be written as \[ x^2+y^2+2gx+2fy=0. \]

Step 2: Use orthogonality with the first circle.
For \[ x^2+y^2+6x-15=0, \] we have \[ g_1=3,\quad f_1=0,\quad c_1=-15. \] The condition of orthogonality is \[ 2gg_1+2ff_1=c+c_1. \] Thus, \[ 6g=-15. \] Hence, \[ g=-\frac52. \]

Step 3: Use orthogonality with the second circle.
For \[ x^2+y^2-8y-10=0, \] we have \[ g_2=0,\quad f_2=-4,\quad c_2=-10. \] Applying the orthogonality condition, \[ -8f=-10. \] Therefore, \[ f=\frac54. \]

Step 4: Form the required circle.
Substituting the values of \(g\) and \(f\), \[ x^2+y^2-5x+\frac52y=0. \] Multiplying throughout by \(2\), \[ 2(x^2+y^2)-10x+5y=0. \]

Step 5: Final conclusion.
Hence the required circle is \[ \boxed{2(x^2+y^2)-10x+5y=0}. \]
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