SAT Mathematics Practice Test 2

The equation of a circle with center (h,k) and radius r is (x−h)2+(y−k)2=r2.To put the equation x2+y2+4x−2y=−1 The equation of a circle with center (h, k) and radius rx2+y2+4x−2y=−1
(x2+4x)+(y2−2y)=−1
(x2+4x+4)−4+(y2−2y+1)−1=−1
(x+2)2+(y−1)2−4−1=−1
(x+2)2+(y−1)2=4=22
Therefore, the radius of the circle is 2.
Choice C is incorrect because it is the square of the radius, not the radius. Choices B and D are incorrect and may result from errors in rewriting the given equation in standard form.