(3a²+4a+5)-(2a²-2a+3)
=a²+6a+2
=(a²+6a+9)-7
=(a+3)²-7
当(a+3)²大于7时,3a²+4a+5大于2a²-2a+3；
当(a+3)²小于7时,3a²+4a+5小于2a²-2a+3.