if a.b=0 then find the angle between two vectors

For dot product of two vectors the following relationships are true:

x . y = (x_{1}, x_{2},...) . (y_{1}, y_{2},...) = \sum {x_{1}y_{1} + x_{2}y_{2}+... = ||x|| ||y|| \cos{\theta}

where \theta is the angle between the vectors and ||a|| is the length of the vector. Using cos^{-1} and assuming non-zero length vectors you can deduce your answer.