f(x)=(x+3)/(x-4),x≠4
=（x-4+7)(x-4)
=1+7/(x-4)
值域f(x)≠1
g(x)=(x^2-9)/(x^2-7x+12),x≠4,x≠3
=(x+3)/(x-4)
g(x)≠1,g(x)≠-6
所以：A包含B