Question

已知向量a,b的夹角为120°,且|a|=3,|b|=1,则|a-2b|=

=根号13？

Answer 1

|a-2b|= √[(a-2b)]^2=√[a^2-4a*b+4b^2]
=√[|a|^2]-4|a|*|b|*cos120°+4|b^2|]
=√[9-4*3*1*(-1/2)+4]=√19

Answer 2

|a-2b|^2=(a-2b)^2=a^2-4ab+4b^2=9-4*3*1*(-1/2)+4*1=19
|a-2b|=√19

Answer 3

更号7