f(x) = x^2lga + 2x + 4lga的最大值为3,
因此,lga < 0. 0 < a < 1.
f(x)的对称轴位于 x = -1/lga,
3 = f(-1/lga) = (-1/lga)^2(lga) + 2(-1/lga) + 4lga = 1/lga -2/lga + 4lga = 4lga -1/lga,
4(lga)^2 - 3lga - 1 = [4lga + 1][lga - 1] = 0,
lga = 1(不符题意,舍去)或者lga = -1/4, a = 10^(-1/4).