6x²-12x+7
=6(x²-2x)+7
=6(x²-2x+1-1)+7
=6(x²-2x+1)-6+7
=6(x²-2x+1)+1
=6(x-1)²+1
因为
(x-1)²>=0
所以6(x-1)²+1>0