(1)y=x^2-2x
y=(x-1)^2-1

(2)y=-x^2+3x-(13/4)
y=-(x^2-3x)-(13/4)
y=-{(x-3/2)^2-(9/4)}-(13/4)
y=-(x-3/2)^2+(9/4)-(13/4)
y=-(x-2/3)^2-1


(3)y=(1/2)x^2+x-3
y=(1/2)(x^2+2x)-3
y=(1/2){(x+1)^2-1}-3
y=(1/2)(x+1)^2-(1/2)-3
y=(1/2)(x+1)^2-(1/2)-(6/2)
Y-(1/2)(x+1)^2-(7/2)

とりあえず載せとく。ミスってるかもしれないけど