Question

设向量│a│=1,向量│b│=2,且向量a与向量b的夹角为120°,则向量│a-2b│=?

Answer 1

|a-2b|^2

=(a-2b).(a-2b)

=|a|^2+4|b|^2- 4|a||b|cos120°

=1+16+4

=21

|a-2b|=√21