text: They can be used (practically) for taking measurements. They also crop up in periodic motion and lots of other areas of mathematics. They are functions of angles. \n\nIt is simplest to see from a right triangle ABC, where the angle B is the 90 degree angle. In this angle the length of the side AB might be called a, the length of the side BC might be called b and the length of the side CA might be called c. (draw it out.)\n\n\nSide c is the hypotenuse.\n\n For angle A, the Opposite side is b. For Angle A, the Adjacent side is a.\n\nFor Angle C, the opposite side is a, the Adjacent side is b.\n\nGiven this triangle, \nthe sin(A) = Opposite/Hypotenuse = b/c.\ncos(A) = Adjacent/Hypotenuse = a/c.\ntan(A) = sin(A)/cos(A) = b/a = Opposite/Adjacent.\n\n\nIf you're measuring height of a building and know how far you are from the building (adjacent side) and the angle you must look up to see the top edge of the building, tou can use (adjacent*tan(angle looking up)) = height of building.