Source: http://www.cafepedagogique.net/communautes/MarcCourbot/Documents/-%20Troisi%C3%A8me%20-/-%20TRIANGLES%20RECTANGLES%20-%20TRIGONOMETRIE%20-/COSINUS%20D%27UN%20ANGLE%20AIGU%20DANS%20UN%20TRIANGLE%20RECTANGLE.pdf