If the surface area of a sphere is 100π cm^2, what is the radius?
a. 10 cm
b. 5 cm
c. 25 cm
d. 3 cm

Surface area of sphere=100pi cm^2
4pi×r^2=100 pi
pi cancel on both sides
4×r^2=100
r^2=100/4
r^2=25
so r=5
So option b 5 cm is correct.

X^2 - 4x +(4/2^2) + y^2 + 8y + (8/2^2) = -4 + (4/2^2) + (8/2^2)(x + 2)^2 + (y + 4)^2 = -4 + 4 + 16your center is (-2, -4) and the radius is the sqrt of -4 + 4 + 16 which is 4