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Cosine: Properties
The cosine function has a number of properties that result from it being periodic and even. Most of these should not be memorized by the reader; yet, the reader should be able to instantly derive them from an understanding of the function's characteristics. The cosine function is periodic with a period of 2p,
which implies that cos(q) = cos(q
+ 2p)
or more generally,cos(q) = cos(q
+ 2pk), k Î integers
The function is even; therefore, cos(q) = cos(q)
Formula: cos(x + y) = cos(x)cos(y)  sin(x)sin(y)
It is then easily derived thatcos(x  y) = cos(x)cos(y) + sin(x)sin(y)
Or more generally,cos(x ± y) = cos(x)cos(y) ±
sin(x)sin(y)
From the above we can easily derive that cos(2x) = cos^{2}(x)  sin^{2}(x)
(The cos^{2}(x) is alternate notation for (cos(x))^{2}.)
By observing the graphs of sine and cosine, we can express the cosine
function in terms of sine: cos(x) = sin(x  p/2)
The pythagorean identity gives an alternate expression for cosine
in terms of sine cos^{2}(x) = 1  sin^{2}(x)
The Law of Cosines relates all three sides and one of the angles of an arbitrary (not necessarily right) triangle: c^{2} = a^{2} + b^{2}  2ab
cos(C).
where A, B, and C are the angles opposite sides a, b, and c respectively.
It can be thought of as a generalized form of the pythagorean theorem.



