∵ a and b are unit vectors |a|=|b|=1 ∵ (a+2b) and 5a−4b are perpendicular. ∴ (a+2b).(5a−4b)=0 ⇒ 5(a.b)−4a.b+10a.b−8(b.b)=0 ⇒ 5|a|2+6a.b−8|b|2=0 ⇒ 5+6a.b−8=0 ⇒ 6a.b=3 ⇒ a.b=
3
6
=
1
2
∵ a.b=|a||b|cosθ θ= angle between a and b ⇒ |a||b|cosθ=