M(x,y)在直线x+2y+1=0上移动,求f=2^x+4^y的最小值

M(x,y)在直线x+2y+1=0上移动,求f=2^x+4^y的最小值
x+2y+1=0
x+2y=-1
2^x+4^y
=2^x+2^(2y)
>=2根号2^(x+2y)
=根号2
x=-1/2 y=-1/4
=2^x+2^(2y)怎么等于>=2根号2^(x+2y)解释一下