12x+y=6 斜率k=-12f’(x)=3x²+2ax-9=3(x²+(2/3)*ax)-9=3(x²+(2/3)ax+a²/3²)-a²/3-9=3(x+a/3)²-a²/3-9x=-a/3时 f’(x)=-a²/3-9最小-a²/3-9=-12a=±3