设向量a=(cos23,cos67),向量b=(cos68,cos22),向量u=向量a+t向量b(t属于R)

a=(cos23, sin23), b=(cos68, sin68)
|a|=|b|=1

1. a*b=cos23cos68+sin23sin68=sin(23-68)
=cod(-45)=cos45=√2/2

2. u=a+tb
|u|^2=(a+tb)^2=a^2+2tab+t^2b^2
=t^2+√2t+1
=(t+√2/2)^2+1/2
故：|u|^2的最小值是1/2
|u|的最小值是√2/2

a*b=cos23*cos68+cos67*cos22
=cos23*cos68+sin23*sin68
=cos45
=1
第二小题我也不太会，呵呵