A,B的横坐标分别为,2根号5/5
得cos a=根号2/10 cos b=2根号5/5
cos(a-b)=cos acos b+sin asin b=根号2/10*2根号5/5+7根号2/10*根号5/5
=根号10/25+7根号10/50
=9根号10/50
2
A （根号2/10,7根号2/10）
cos=根号2/10*c1+7根号2/10*c2/（1*1）=2/根号2
c1^2+c2^2=1
得c1=0.8
c2= 0.6
或
c1=-0.6
c2= 0.8
C(0.8,0.6)或(-0.6,0.8)