What is f(x)=8x2+4x written in vertex form

f(x)=8(x^2+1/2x)
using completing the square:
f(x)=8(x^2+1/2x+1/16)-(8*1/16) -->since we added a 1/16 inside the parenthesis, we need to subtract that quantity times 8
f(x)=8(x+1/4)-1/2