(1) |a|^2=a*a=(2m+n)(2m+n)=4mm+4mn+nn=4+4根号2根号2|2+2=10
　　|b|^2=bb=(-3m+n)^2=9mm-6mn+nn=9-6+2=5
　　ab=(2m+n)(-3m+n)=-6mm-mn+nn=-6-1+2=-5
　　cos(a,b)=-5/根号50=-根号2/2
　　a,b夹角是135度
(2) c=ta-b d=2m-n
　　cXd=(ta-b)X(2m-n)=(2tm+tn+3m-n)X(2m-n)=((2t+3)m+(t-1)n)X(2m-n)
=-(2t+3)mXn+2(t-1)nXm
=(2t+3+2t-2)nXm=0向量
　　4t+1=0 t=-1/4