若平面向量a,b,满足|a+b|=1,a+b平行于x轴,向量b=(2,-1),则向量a=

设向量a=（x,y）|a+b|=1（a+b）²=1a²+2a·b+b²=1a²+2a·b+5=1a²+2a·b=-4a+b=（2+x,y-1）‖x轴则a+b斜率为0y-1/2+x=0,y=1,且x≠-2a²=x²+12a·b=2（2x-1）=4x-2x²+1+4x-2=-4...