Cosine is co added onto sine. Basically, cosine is the sine function moved over 90degrees or pi/2 (pi/2 on a unit circle is 90 degrees)
Sin(x)=cos(x+90) <--degrees
Sin(x)=cos(x+pi/2) <--radians
The above two equations for converting them is called a cofunction identity. There's many more identities to convert sines, cosines, tangents, cosecantes, secantes, and cotangents between each other. This is taught to you in PreCalculus.