Cosine

The functions sine and cosine take values between -1 and 1 (-1 and 1 included).

"Eh? Is the tangent cosine divided by sine?" "Other way round!"

The functions sine and cosine have a range of values between -1 and 1, inclusive.

The rational parametrisation of a circle is almost always far superior to the usual parametrisation in terms of cosine and sine.

The trigonometric functions sine, cosine, and tangent relate the various sides and angles of a triangle.

The sine and cosine functions both satisfy this property.