5x^2-6x+10+4/5
=5(x^2-6x/5)+10+4/5
=5(x^2-6x/5+9/25)-9/5+10+4/5
=5(x-3/5)^2+9
因为5(x-3/5)^2>=0,
所以5(x-3/5)^2+9>0
即5x^2-6x+10+4/5>0