sin x = sin (180° - x)
所以,
sin (170° - a) = sin [180° - (170° - a)]
= sin (10° + a)
根据方程式,
sin(u ±v) = sin u cos v ± cos u sin v
sin (10° + a) cos (70° + a) - cos (10° + a) sin (70° + a)
= sin [(10° + a) - (70° + a)]
= sin - 60°
= - sin 60°
= -(√3)/2
= -0.866
纯手打,蒙采纳,谢谢~~