Question

已知向量a=(-2,-1)b=(x,1).x属于R.当x=3时,一：求ab的积和ab相加的绝对值.二：若的夹角余弦为正.

Answer 1

(1)ab = -2x + (-1)*1 = -2x - 1 = -7

|a+b| = |(x-2,0)| = |x-2| = 1

(2)cos=ab/(|a|*|b|)=(-2x-1)/√5(x^2+1)>0

解得x范围是x < -1/2