From the Law of Cosines
Cos (C) = (a^2 + b^2 -c^2) / 2 * a * b
Cos (C) =(40^2 + 41^2 -9^2) / 2 * 40 * 41
Cos (C) = 1,600 + 1,681 - 81 / 3,280
Cos (C) = 3,200 / 3,280
Cos (C) = <span> <span> <span> 0.9756097561 </span> </span> </span>
Incidentally, Angle C = 12.68 Degrees
Cos (C) = (a^2 + b^2 -c^2) / 2 * a * b
Cos (C) =(40^2 + 41^2 -9^2) / 2 * 40 * 41
Cos (C) = 1,600 + 1,681 - 81 / 3,280
Cos (C) = 3,200 / 3,280
Cos (C) = <span> <span> <span> 0.9756097561 </span> </span> </span>
Incidentally, Angle C = 12.68 Degrees