Question

已知向量a与b的夹角为120°,a的模=3,a加b的模=√13,则b的模=?

Answer 1

|a+b|=√13

|a+b|^2=13

|a|^2+|b|^2+2a.b=13

a.b=3|b|(-1/2)

9+|b|^2-3|b|=13

|b|^2-3|b|-4=0

|b|=4