(1)向量a+b的模=√（a+b）^2=√（a+b）^2
=√(a^2+b^2+2ab）=√(9+16+2*3*4*1/2）=√37.
向量a-b的模=√（a-b）^2=√（a-b）^2
=√(a^2+b^2-2ab）=√(9+16-2*3*4*1/2）=√13.
(2)向量a+b与a-b的夹角@的余弦值 =(a-b)*(a+b)/(|a-b||a+b|)
=(a^2-b^2)/(|a-b||a+b|)=-7/√(37*13).