三角學的應用

cos77°- (cos13°/tan77°)
=cos77°- [cos13°/(sin77°/cos77°)]
=cos77°- [(cos13°cos77°)/sin77°]
=cos77°- [(cos13°cos77°)/cos(90°-77°)]
=cos77°- [(cos13°cos77°)/cos13°]
=cos77°- cos77°
=0

2014-05-18 15:27:44 補充：
解釋:

tan77°=sin77°/cos77°

sin77°=cos(90°-77°)=cos13°

SO
tan77°=cos13°/cos77°

按小算盤就計到，有什麼難度﹖以前沒有小算盤就要查對數表。