f（x）=2（a+b）b
=2(sinx+cosx,-1/4).(cosx,-1)
=2(sinx+cosx)cosx+2*1/4
=2sinxcosx+2cos²x+1/2
=sin2x+cos2x+1+1/2
=√2sin(2x+π/4)+3/2=2(sinx+cosx,-1/4).(cosx,-1)第一步是怎么得来的？谢谢！a+b的横坐标=a的横坐标+b的横坐标 a+b的纵坐标=a的纵坐标+b的纵坐标