已知向量a.b的夹角为120度，|a|=1,|b|=3则|5a-b|=

| 5a-b |^2==(5a-b)*(5a-b)=25|a|^2+|b|^2-10a*b=25+9-30cos120=34+15=49, |5a-b|=7.