Through the sides and the angle between them

The third method uses to find the area of the parallelogram of its side and the sine of the acute angle between them. This logically follows from the first method, if the height is unknown. Proof: the height in the parallelogram always forms a right-angled triangle on one of the sides, in which trigonometric relations act. The sine of the angle, by definition, is the ratio of the opposite side to the hypotenuse. Thus, we obtain the above formula.