设单位向量a=(x,y)b=(2,-1)若a//b 则 | x+3y |=?

a//b==>
x/2=y/(-1)
所以,
x= - 2y
x+2y=0
|x+3y|=|y|
x= -2y
a=(-2y,y)
4y^2+y^2=5y^2=1
y^2=1/5
|y|=√5/5
|x+3y|=√5/5