In a quadrilateral ABCD, −−→AC is the bisector the angle between −−→AB and −−→AD which is 2π3. If 15|−−→AC|=3|−−→AB|=5|−−→AD|, then cosine of the angle between −−→BA and −−→DC is (correct answer + 1, wrong answer - 0.25)

In a quadrilateral ABCD, −−→AC is the bisector the angle between

1.	In a quadrilateral ABCD, −−→AC is the bisector the angle between −−→AB and −−→AD which is 2π3. If 15\|−−→AC\|=3\|−−→AB\|=5\|−−→AD\|, then cosine of the angle between −−→BA and −−→DC is (correct answer + 1, wrong answer - 0.25)
Answer» In a quadrilateral $A B C D$ , $- - \to A C$ is the bisector the angle between $- - \to A B$ and $- - \to A D$ which is $\frac{2 π}{3}$ . If $15 \| - - \to A C \| = 3 \| - - \to A B \| = 5 \| - - \to A D \|$ , then cosine of the angle between $- - \to B A$ and $- - \to D C$ is (correct answer + 1, wrong answer - 0.25)