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Circles

SAT Math· difficulty 4/5

The equation x2+y2+4x−12y−9=0x^2 + y^2 + 4x - 12y - 9 = 0 represents a circle. What is the radius?

  • A

    49\sqrt{49}

  • B

    33

  • C

    77

    check_circle
  • D

    9\sqrt{9}

Explanation

Complete the square: (x+2)2−4+(y−6)2−36−9=0(x+2)^2 - 4 + (y-6)^2 - 36 - 9 = 0, so (x+2)2+(y−6)2=49(x+2)^2 + (y-6)^2 = 49. Radius =7= 7.

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