Answer
a)$0.$
b)$90^\circ$.
c)orthogonal.
Work Step by Step
By definition the dot product of 2 vector $ai+bj$ and $ci+dj$ is: $(ai+bj)(ci+dj)=ac+bd.$ If this product is 0, then the vectors are orthogonal (perpendicular) and hence the enclosed angle is $90^\circ.$
a)$v\cdot w=(2i+j)(i-2j)=2\cdot1+1\cdot(-2)=0.$
b)From above the enclosed angle is:$90^\circ$.
c)From above they are orthogonal.
