3x^2-5x-12
=3(x^2-5/3x+25/36)-3×25/36-12
=3(x-5/6)^2-169/12
当x=5/6时3x^2-5x-12有最小值-169/12
-3x^2-5x-12
=-3(x^2+5/3x+25/36)+3×25/36-12
=-3(x+5/6)^2-119/12
当x=-5/6时3x^2-5x-12有最大值-119/12
3x^2-5x-12
=3(x^2-5/3x+25/36)-3×25/36-12
=3(x-5/6)^2-169/12
当x=5/6时3x^2-5x-12有最小值-169/12
-3x^2-5x-12
=-3(x^2+5/3x+25/36)+3×25/36-12
=-3(x+5/6)^2-119/12
当x=-5/6时3x^2-5x-12有最大值-119/12