函数f(x)=ln(x2-4x-12）递减区间就是函数g(x)=x^2-4x-12的递减区间.
g(x)=(x-2)^2-16,对称轴是X=2,开口向上,递减区间是(-无穷,2]
又定义域是x^2-4x-12>0
(x-6)(x+2)>0
即有x>6或x