向量a,b满足|a|=2,ab=3/2,|a+b|=2根号2，则向量夹角的余弦值为

|a+b|=2√2 (a+b)^2=8 a^2+2ab+b^2=8 4+3+b^2=8 b^2=1 |b|=1 cos=(a*b)/(|a||b|)=(3/2)/(2*1)=3/4