It is known that a parallelogram has rotational symmetry, rotated 180 degrees about it's center point. Now by definition a parallelogram is a quadrilateral with two pairs of parallel sides, hence it's name. If you were to take a look at the attachment below, you might see the connection.
Rotational Symmetry will make it so that side AB corresponds to CD, respectively AD and CB. The sides will coincide with one another after a 180 degree rotation, so that AB = CD, and AD = CB. Hence, in the same parallelogram, opposite sides are congruent.
In order to cross multiply you have to multiply the numerator and denominator of the first fraction by the bottom number of the second fraction and see if the equation is still true:
In quadratic equations of real coefficients, the complex roots always occur in conjugate bases. It means, if, 2 + 3i is one of the roots and then the second root must be 2 - 3i.