向量CA=CB+BA=-(-2a+8b)-(a+5b)=a-13b=x向量CB+y向量CD=x(2a-8b)+3y(a-b)=(2x+3y)a-(8x+3y)b,a,b不共线，∴2x+3y=1,8x+3y=13,解得x=2,y=-1.∴x+y=1.
