3x²-5x+11
=3(x²-5x/3)+11
=3(x-5/6)²+11-25/12
=3(x-5/6)²+107/12
因为(x-5/6)²≥0
所以：3(x-5/6)²+107/12>0
所以：3x²-5x+11恒大于0