【導(dǎo)讀】已知兩個(gè)非零向量),(11yxa??,這就是說(shuō):兩個(gè)。對(duì)于任意的實(shí)數(shù)k,向量k(,)xy?的有向線段的起點(diǎn)和終點(diǎn)的坐標(biāo)分別為),,(22yx,那么。a=,b=,則(a+b)·(a-b)=_____. 例A(1,2),B(3,4),C(5,0),求∠BAC的正弦值。xOy中,若定點(diǎn)A(1,2)與動(dòng)點(diǎn)P(x,y)滿足OPOA?則點(diǎn)P的軌跡方程是___________.在OC上是否存在點(diǎn)M,使MAMB?