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One answer is that the cosine function can be defined as follows:

cos(x) = 1 - x2/2! + x4/4! + x6/6! - ... where x is the measure of the angle measured in radians.

Each term in this infinite series is an even power of x and is therefore even. And the sum of even functions is even.

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Q: Why is cosine even?
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Related questions

Which trigonometric functions are even?

Cosine and secant are even trig functions.


Why is cos an even function?

An even function is one where f(x) = f(-x) For cosine, cos(x) = cos(-x), thus cosine is an even function.


What is cosine cubed?

It is cosine*cosine*cosine.


Is it true or false that the cosine function is an odd function?

False; the cosine function is an even function as cos(-x) = -cos(x).


Why is the secant function is an even function and the tangent and cosecant are odd functions?

I find it convenient to express other trigonometric functions in terms of sine and cosine - that tends to simplify things. The secant function is even because it is the reciprocal of the cosine function, which is even. The tangent function is the sine divided by the cosine - an odd function divided by an even function. Therefore it is odd. The cosecant is the reciprocal of an odd function, so it is naturally also an odd function.


What is the difference between negative sine and cosine graphs and positive sine and cosine graphs?

The negative sine graph and the positive sine graph have opposite signs: when one is negative, the other is positive - by exactly the same amount. The sine function is said to be an odd function. The two graphs for cosine are the same. The cosine function is said to be even.


What is the cosine of 3.14?

what is the cosine of 3.14 ?


What is cosine of 1?

Cosine of 1 degree is about 0.999848. Cosine of 1 radian is about 0.540302.


How does the tangent function relate to sine and cosine?

Tangent = sine/cosine provided that cosine is non-zero. When cosine is 0, then tangent is undefined.


What is the cosine of 6 degrees?

Cosine is 0.995


What is an inverse cosine?

The inverse of the cosine is the secant.


Reciprocal of a cosine?

Reciprocal of Cosine is Secant