Question

若已知向量A,B满足向量A的模等于1,向量B的模等于2,向量A减B的模等于2,求向量A加B的模?

Answer 1

解法1:画个平行四边形,再用余弦定理.

解法2:

(a-b)^2=a^2+b^2-2abcosC=4

2abcosC=1

(a+b)^2=a^2+b^2+2abcosC=6

|a+b|=根号6