Question

若向量a,b满足|a|=1,|b|=2,且a与b的夹角为π/3,则|a加b|=?

Answer 1

|a+b|^2 =(a+b).(a+b) = |a|^2+|b|^2 + 2abcosπ/3=1+4+2=7
|a+b|=√7

Answer 2

|a+b|
=√(|a+b|)²
=√(a²+b²+2|a||b|cosπ/3}
=√(1+4+2)
=√7