I have a triangle with two vertices known (1,3) and (-4,1).

I know a formula relating the acute angle formed by two lines and their slopes. It is

$\displaystyle tanA=\lvert\frac{m1-m2}{1+m1m2}\rvert$

I used this formula for the hypotenuse and one of the sides(m1) and then again used this formula for the hypotenuse and the other side(m2). I used the result m1m2=-1 in the second result and got two values of m1. But when I equate those values I get tanA=0.

What is wrong?