已知向量a,b满足:IaI=1,IbI=2,Ia-bI=根号七,求Ia-2bI

|a-b|²=7
a²+b²-2a.b=7
1+4-2a.b=7
a.b= -1
Ia-2bI²
=a²-4a.b+4b²
=1+4+4*4
=21
|a-2b|=√21