If the endpoints of the diameter of a circle are (−10, −8) and (−6, −2), what is the standard form equation of the circle? a) (x − 8)2 + (y − 5)2 = 13 b) (x + 8)2 + (y + 5)2 = 13 c) (x − 8)2 + (y − 5)2 = 13 d) (x + 8)2 + (y + 5)2 = 13

Selections B and D both appear to be appropriate.
  B) (x+8)² + (y+5)² = 13
  D) (x+8)² + (y+5)² = 13


The center is at the midpoint of the diameter, ((-10-6)/2, (-8-2)/2) = (-8, -5). For center (h, k) and radius r, the equation is
  (x -h)² +(y -k)² = r²
  (x -(-8))² +(y -(-5))² = (√13)²
  (x+8)² + (y+5)² = 13