Originally Posted by **Joseph3** I know the angle is 90 degrees. and the formula to calculate the angle of vector A is inverse tan of (Ay / Ax), but Excel returns an error message because Ax is zero.
Thanks. |

It has to do with, as you say, the 90 degree angle.

Try this:

$\displaystyle \lim_{A_x \to 0^-} tan^{-1} \left ( \dfrac{A_y}{A_x} \right )$

and

$\displaystyle \lim_{A_x \to 0^+} tan^{-1} \left ( \dfrac{A_y}{A_x} \right )$

What do these two limits tell you?

-Dan