It is a non-linear function of theta.
If sine theta is 0.28, then theta is 16.26 degrees. Cosine 2 theta, then, is 0.8432
It is 1.
For such simplifications, it is usually convenient to convert any trigonometric function that is not sine or cosine, into sine or cosine. In this case, you have: sin theta / sec theta = sin theta / (1/cos theta) = sin theta cos theta.
The sine and cosine of complementary angles are related through the identity (\sin(90^\circ - \theta) = \cos(\theta)) and (\cos(90^\circ - \theta) = \sin(\theta)). This means that the sine of an angle is equal to the cosine of its complementary angle, and vice versa. Therefore, for any angle (\theta), the values of sine and cosine are essentially swapped when considering complementary angles.
Tangent (theta) is defined as sine (theta) divided by cosine (theta). In a right triangle, it is also defined as opposite (Y) divided by adjacent (X).
Cosine squared theta = 1 + Sine squared theta
If sine theta is 0.28, then theta is 16.26 degrees. Cosine 2 theta, then, is 0.8432
Since secant theta is the same as 1 / cosine theta, the answer is any values for which cosine theta is zero, for example, pi/2.
It is 1.
cosine (90- theta) = sine (theta)
For such simplifications, it is usually convenient to convert any trigonometric function that is not sine or cosine, into sine or cosine. In this case, you have: sin theta / sec theta = sin theta / (1/cos theta) = sin theta cos theta.
The sine and cosine of complementary angles are related through the identity (\sin(90^\circ - \theta) = \cos(\theta)) and (\cos(90^\circ - \theta) = \sin(\theta)). This means that the sine of an angle is equal to the cosine of its complementary angle, and vice versa. Therefore, for any angle (\theta), the values of sine and cosine are essentially swapped when considering complementary angles.
It is cotangent(theta).
The cosine of theta is adjacent over hypotenuse, given a right triangle, theta not being the 90 degree angle, adjacent not being the hypotenuse, and theta being the angle between adjacent and hypotenuse. In a unit triangle, i.e. in a unit circle circumscribed with radius one, and theta and the center of the circle at the origin, cosine of theta is X.
If X and Y are sides of a right triangle, R is the hypoteneuse, and theta is the angle at the X-R vertex, then sin(theta) is Y / R and cosine(theta) is X / R. It follows, then, that X is R cosine(theta) and Y is R sin(theta)
There is no easy simplification.
An even function is one where f(x) = f(-x) For cosine, cos(x) = cos(-x), thus cosine is an even function.