The answers are a=1 and a=49/23
However, I do not understand why a=1 is an accepted answer (Because If we substituted a=1 in the scalar product of the two direction vectors, it gives a negative answer so cos(angle) = -ve ----->The angle is obtuse??? (The question stated that the angle is acute..)
Here is the explanation that was provided: (but I don't understand it..) Can anyone please help?
+9519 It is correct since the angle could be \(180^\circ - \cos^{-1}\left(\dfrac16\right)\). Having a \(x^\circ\) angle is basically the same thing as having a \((180 - x)^\circ\) angle.
Sorry, but how is having a "x" angle the same as 180-x ?
(If x=60 then it is acute, but 180-60 = 120 is obtuse?)
