设函数f(x)=xx+2x>0,观察:f1(x)=f(x)=xx+2,f2(x)=f(f1(x))=x3x+4,f3(x)=f(f2(x))=x7x+8,f4(x)=f(f3(x))=x15x+16, ……根据以上事实,由归纳推理可得当n∈N*且n≥2时,fn(x)=f(fn-1(x))= ( )
设实数x,y满足:x+3y+5≥0x+y-1≤0x+2≥0,则z=2x+4y的最小值是( )