Length of belt

[ggauny@live.fr]

For the son of my daugther, who is studying elementari trigonometric calculus, at low school (not hight school), length of belt with 2 pulleys, i said do this :
L=Pi(D2)+(D2-D1)(TAN(#)-#) where #=ACOS(D2-D1)/(2C). This formula was edited in HP journal of NOV 1976 about HP 91. Yes i am not "new in box". my first HP is HP 25 not c. I give my hello to Master mister Horn, i have learned with profit "one-minute marvels" and else.