左边=3 - (x+1)/(x^2+x+1)
=3 - (x+1)/[(x+1)^2 - (x+1) + 1]
=3 - 1/[(x+1) + 1/(x+1) -1]
|(x+1) + 1/(x+1)|≥2
所以-1/3≤1/[(x+1) + 1/(x+1) -1]≤1且1/[(x+1) + 1/(x+1) -1]≠0
所以左端最小值为2
要大于n
所以n=1