(1)
|a|=1,|b|=2
a.b =|a||b|cosx
(1,0).(1,√3)=1(2)cosx
1=2cosx
x= 60°
(2)
(ka+b).(a-2b)=0
k|a|^2-2|b|^2+(-2k+1)|a||b|cos60°=0
k-8+(-2k+1)=0
k=-7