The standard form of the equation of a parabola is
y= 1/2x^2 - 4x+21. What is
the vertex form of the equation?
A. y = 1/2(x+4)^2 +13
B. y= 1/2(x+4)^2 +21
C. y = 1/2(x-4)^2+21
D. y= 1/2(x-4)^2+13​