若f(x)=3x+4,f1(x)=f(x),f2(x)=f(f1(x)),f3(x)=f(f2(x)),···,···,fn(x)=f(fn-1(x)),n属于N*,则
问题描述：
若f(x)=3x+4,f1(x)=f(x),f2(x)=f(f1(x)),f3(x)=f(f2(x)),···,···,fn(x)=f(fn-1(x)),n属于N*,则
f(f(···f(x)))=?注：共n个
答
(3^n)x+4+4*3+4*3^2+...+4*3^(n-1)
=(3^n)x + (4-4*3^(n-1))/(1-3)
=(3^n)x + 2*3^(n-1)-2