This calculator computes the diagonals of a parallelogram and adjancent angles from side lengths and angle between sides.
It is done with the help of law of cosines
Check the picture. First, we use law of cosines to find out d1, then we find second angle of parallelogram, which is , then we again use law of cosines to find out d2. Remaining angles are found from the same law of cosines. After that we find diagonal intersection angles using the fact that sum of triangle angles is 180.