3x²+3x+1
=3(x²+x+1/4-1/4)+1
=3(x²+x+1/4)-3/4+1
=3(x+1/2)²+1/4
所以最小值是1/4
-4x²+2x+3
=-4(x²-x/2+1/16-1/16)+3
=-4)x²-x/2+1/16)+1/4+3
=-4(x-1/4)²+13/4
所以最大值是13/4