y=3x^2 -2x+1
=3[x^2 -(2/3)x]+1
=3[x^2 -(2/3)x+(1/9)-(1/9)]+1
=3[x-(1/3)]^2 +(2/3)
y=(1/2)x^2 -x-3
=(1/2)[x^2 -2x-6]
=(1/2)[x^2 -2x+1-7]
=(1/2)(x-1)^2 -(7/2)