y=x平方—2x+2 =(x²-2x+1)+1 =(x-1)²y=1/2x平方+2x+5 =1/2(x²+4x+4)+3 =1/2(x+2)²+3 y=ax平方+bx+c =a(x²+bx/a+b²/4a²)-b²/4a²+c =a(x+b/2a)²+(4ac-b²)/4a²
