已知向量a,b满足| a |=1 b=(2,1)且λ a+b=0 则 |λ |=

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已知向量a,b满足| a |=1 b=(2,1)且λ a+b=0 则 |λ |=
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已知向量a,b满足| a |=1 b=(2,1)且λ a+b=0 则 |λ |=
已知向量a,b满足| a |=1 b=(2,1)且λ a+b=0 则 |λ |=

已知向量a,b满足| a |=1 b=(2,1)且λ a+b=0 则 |λ |=
设a=(x,y) x^2+y^2=1 入x+2=0 入y+1=0
=> 入=-2/x=-1/y => 4y^2+y^2=1 => y=±√5/5
∴ λ=±√5 => |λ|=√5