## Addition and Subtraction Formulas for Sine and Cosine IV

$2\cos\alpha\cos\beta=\cos (\alpha+\beta)+\cos (\alpha-\beta)\\ 2\sin\alpha\cos\beta=\sin (\alpha+\beta)+\sin (\alpha-\beta)$

**Summary**. We present a wordless proof of two basic trigonometric identities involving two angles, sine, and cosine.

### References

- J. Molokach, , The College Mathematics Journal, MAA, May 2016 (47), n 3, 199

### Trigonometry

- What Is Trigonometry?
- Addition and Subtraction Formulas for Sine and Cosine
- Sine of a Sum Formula
- Addition and Subtraction Formulas for Sine and Cosine II
- Addition and Subtraction Formulas for Sine and Cosine III
- Addition and Subtraction Formulas for Sine and Cosine IV
- Addition and Subtraction Formulas

- The Law of Cosines (Cosine Rule)
- Cosine of 36 degrees
- Tangent of 22.5
^{o}- Proof Wthout Words - Sine and Cosine of 15 Degrees Angle
- Sine, Cosine, and Ptolemy's Theorem
- arctan(1) + arctan(2) + arctan(3) = π
- Trigonometry by Watching
- arctan(1/2) + arctan(1/3) = arctan(1)
- Morley's Miracle
- Napoleon's Theorem
- A Trigonometric Solution to a Difficult Sangaku Problem
- Trigonometric Form of Complex Numbers
- Derivatives of Sine and Cosine
- ΔABC is right iff sin²A + sin²B + sin²C = 2
- Advanced Identities
- Hunting Right Angles
- Point on Bisector in Right Angle
- Trigonometric Identities with Arctangents
- The Concurrency of the Altitudes in a Triangle - Trigonometric Proof
- Butterfly Trigonometry
- Binet's Formula with Cosines
- Another Face and Proof of a Trigonometric Identity
- cos/sin inequality
- On the Intersection of kx and |sin(x)|
- Cevians And Semicircles
- Double and Half Angle Formulas
- A Nice Trig Formula
- Another Golden Ratio in Semicircle
- Leo Giugiuc's Trigonometric Lemma
- Another Property of Points on Incircle
- Much from Little
- The Law of Cosines and the Law of Sines Are Equivalent
- Wonderful Trigonometry In Equilateral Triangle
- A Trigonometric Observation in Right Triangle
- A Quick Proof of cos(pi/7)cos(2.pi/7)cos(3.pi/7)=1/8

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