用配方法求:(1)2x²-6x+1的最小值,(2)-3x²+5x-1的最大值

2x²-6x+1
=2(x²-3x)+1
=2(x²-3x+9/4)+1-9/2
=2(x-3/2)²-7/2
当 x=3/2时 有最小值为 -7/2
-3x²+5x-1
=-3(x²-5x/3)-1
=-3(x²-5x/3+25/36)-1+25/12
=-3(x-5/6)²+13/12
当 x=5/6时 有最大值为 13/12