Because that is accepted definition. The sine is opposite over hypotenuse, or Y in the unit circle. The cosine is adjacent over hypotenuse, or X in the unit circle. The tangent is sine over cosine, etc. For more information, please see the related link below.
A function that depends on the value of an angle. One way to define it is with a unit circle (a circle with center in the coordinate origin, and radius of 1). To the right is zero, from there, a positive angle is counterclockwise. In this case, the sine is simply the y-coordinate, and the cosine is the x-coordinate of the point on the circle where the ray of the angle crosses the circle. The value of the sine (and cosine) obviously depends on the angle - that's why it is considered a "function". Sine, cosine, tangent, cotangent, cosecans, and secans can also be defined via right triangles; for more details see here: http://en.wikipedia.org/wiki/Sine#Sine.2C_cosine_and_tangent
Basically, it IS a curve.
Yes, but only if the argument of the sine function is in radians.
Repetitive behavior can be described by a point moving in a circle. The time of repetition is equivalent to time taken by that particle to complete that circle. When the point moves in a circle, its angle changes from 0 to 360 degrees; all of these values can be given by a sine function or a cosine function.
Because that is accepted definition. The sine is opposite over hypotenuse, or Y in the unit circle. The cosine is adjacent over hypotenuse, or X in the unit circle. The tangent is sine over cosine, etc. For more information, please see the related link below.
Make a diagram. One way to define sine and cosine is with the unit circle - a circle with a radius of 1 unit. For any point on the circle, the sine is the y-component, while the cosine is the x-component.
A function that depends on the value of an angle. One way to define it is with a unit circle (a circle with center in the coordinate origin, and radius of 1). To the right is zero, from there, a positive angle is counterclockwise. In this case, the sine is simply the y-coordinate, and the cosine is the x-coordinate of the point on the circle where the ray of the angle crosses the circle. The value of the sine (and cosine) obviously depends on the angle - that's why it is considered a "function". Sine, cosine, tangent, cotangent, cosecans, and secans can also be defined via right triangles; for more details see here: http://en.wikipedia.org/wiki/Sine#Sine.2C_cosine_and_tangent
The sine of 180 degrees is 0. Remember, the sine value on a unit circle is the y-value. If you find f(pi) in the function f(x)=sin(x), you will get zero as an answer.
See related links for information about sine charts.
Basically, it IS a curve.
In a circle that has a radius of one you use Pythagorean theorem to derive the sine, cosine and tangent formulas. Draw a circle around the origin on graph paper. The sine is the line segment from the point where the side of the angle intersects down to the x-axis. etc.
Yes, but only if the argument of the sine function is in radians.
Repetitive behavior can be described by a point moving in a circle. The time of repetition is equivalent to time taken by that particle to complete that circle. When the point moves in a circle, its angle changes from 0 to 360 degrees; all of these values can be given by a sine function or a cosine function.
One is the inverse of the other, just like the arc-sine is the inverse of the sine, or division is the inverse of multiplication.
The properties of the cosine and sine function are based on the X and Y coordinates of a point on a circle that has a radius of 1 and a center at the origin (X=0,Y=0). If the angle of the line from the origin to the edge of the circle, at any arbitrary point (X,Y), with respect to the X axis is theta, then sine(theta) is X, and cosine(theta) is Y.
The sine is almost equal to the angle, in case the angle is expressed in radians. Please make a picture of a circle, put a point on the circle at a small angle (say, 10 degrees or less), then draw the sine (a vertical line from the x-axis up to your point) for a small angle. You will see that the arc of the circle has almost the same length as the vertical line you drew. The arc is the angle; the vertical line is the sine.