what is the definition of cosine , sine I know that
sine is the ratio of the perpendicular to the hypotenuse of an acute angle. Similarly cosine is the ratio of the base and hypotenuse .
But now I found that there is sine and cosine of an angle over 90 degrees or negative degrees .
I do not understand how they can determine the value of these sine and cosine .
I have heard about the unit circle , and I find it is not convincing to determined the value of sine and cosine just by looking at the picture . I mean , is there any others method like using equations or theorems to calculate the cosine of an over 90 degrees?
 A: Suppose that the black dot on the rear wheel (where the wheel connects with the coupling rod) was a distance of $1$ from the center of the wheel, and its initial position was directly to the right of the center of the wheel.
Given that the wheel had turned and angle of $\theta$ (from its initial position),
Vertically, how far above the center of the wheel would the black dot be?
Horizontally, how far to the right of the the center of the wheel would the black do be (a negative value would mean that it was to the left)?

Try drawing some diagrams of the wheel in different positions (drawing reference triangles would be helpful here).
Also try drawing a triangle with a hypotenuse of $1$. If $\theta$ is known, what would the length of the opposite side be? What would the length of the adjacent side be?
A: There are many definitions, but geometrically I believe the unit-circle definition is the best. It allows you to understand easly many trigonometric relations. 
A: When you press the cos button on your calculator it probably takes use of the following definition:
cos(x) = $\sum_{0}^{\infty}\frac{(-1)^n x^{2n}}{2n!}$
