y＝x²-2x-3＝x²-2x+1-4＝（x-1）²-4
y=x²-2x-3=x²-2x+1-4=(x-1)²-4
y＝x&78;-2x-3 ＝x&78;-2x+1-4 ＝（x-1）&78;-4
y=ⅹ^2-2ⅹ+1-4=(x-1)^2-4
