Sine of a Sum Formula

The applet below illustrates a proof without words due to Volker Priebe and Edgar A. Ramos [Nelsen, p. 40].

The rhombus inscribed into a rectangle has side length of 1. The rhombus cuts of the rectangle two pairs of equal right triangles. The acute angles of the triangles are A, 90° - A, B, 90° - B. The vertices of the rhombus split the sides of the rectangle into segments of lengths cos A, sin A, cos B, sin B, as shown.

The area of the rhombus is sin(A + B). In the right part og the applet, the triangles rearranged leaving two rectangles unoccupied. The are of the one is sin A × cos B, that of the other cos A × sin B, proving the "sine of the sum" formula

References

1. R. B. Nelsen, Proofs Without Words II, MAA, 2000

Trigonometry

• Sine of a Sum Formula
• Addition and Subtraction Formulas for Sine and Cosine
• Addition and Subtraction Formulas for Sine and Cosine II
• The Laws of Cosines
• The Law of Sines and Cosines
• Cosine of 36 degrees
• Sine, Cosine, and Ptolemy's Theorem