Derivation of the cosine sum identity using geometric methods
There are multiple methods for derivation of cosine difference identity. Once this identity is present, it's easy to derive other identities. For example, the cosine sum identity is another frequently used identities to solve trigonometric problems. Given the cosine difference identity,
\cos(\alpha -\beta ) = \cos \alpha\cos \beta +\sin \alpha \sin \beta
Let \beta be the minus of \theta, the above equation is converted to,
\cos(\alpha -(-\theta) ) = \cos \alpha\cos(-\theta) +\sin \alpha \sin(-\theta)
Using symmetry identities, the above expression is transformed to the identity for cosine of two angles addition,
\cos(\alpha +\theta ) = \cos \alpha\cos\theta -\sin \alpha \sin \theta
What about any method using geometric shapes for proof of cosine sum identity? We will explore the methods in this article.
Using distance formula
In the figure is a circle whose radius is of measure of 1.
Angle \alpha and angle -\beta share a common arm, which coincides with axis x in the right direction. A, B, D are intersection points of their arms with the unit circle.
Angle \beta shares the same arm with the other arm of \alpha and intersects the circle at point C on the other arm.
Since OA,OB, OC, OD are the radius of the circle with measure of 1, the coordinates of point A,B, C, D are as follow,
A (1,0),
B (\cos\alpha, \sin\alpha),
C(\cos(\alpha+\beta),\sin(\alpha+\beta) ),
D (\cos\beta,-\sin \beta )
Using the distance formula, the length of chords AC and RD is expressed below,
BD^2 = (\cos\alpha-\cos\beta)^2 +(\sin\alpha+\sin \beta)^2
Since chords AC and RD subtend the central angle of the same measure \alpha + \beta
AC = BD
Expression (1) is equal to expression (2)
2-2\cos(\alpha+\beta) = 2 - 2(\cos\alpha\cos\beta- \sin\alpha\sin \beta)
Simplify the equation, we get the cosine sum identity.
\cos(\alpha+\beta) = \cos\alpha\cos\beta- \sin\alpha\sin \beta
Using right triangles
In the figure,
\alpha = \angle BAC
\beta = \angle CAD
Construct perpendicular DE to AB
Construct perpendicular DF to AC, FG to AB and FH to DE
Since \triangle AEI and \triangle DFI are right triangles with vertical angles,
\angle FDH = \alpha
\cos(\alpha +\beta ) = \dfrac{AE}{AD}
\because EG = HF
\therefore \dfrac{AE}{AD} = \dfrac{AG-EG}{AD} =\dfrac{AG-HF}{AD} = \dfrac{AG}{AD}-\dfrac{HF}{AD}
= \dfrac{AG}{AF}\cdotp \dfrac{AF}{AD} -\dfrac{HF}{DF}\cdotp \dfrac{DF}{AD}
\because \dfrac{AG}{AF}=\cos \alpha, \dfrac{AF}{AD} = \cos \beta, \dfrac{HF}{DF} = \sin \alpha, \dfrac{DF}{AD} =\sin \beta
\therefore \cos(\alpha +\beta ) =\cos \alpha \cos \beta - \sin \alpha \sin \beta
This is the end of our proof for the cosine sum identity.
Steven Zheng
posted 3 years ago