高中物理學到的三角函數?

1 Tan280°為何=－Tan80°呢
Sol
Tan280°
=Tan(360°－80°)
=－Tan80°
2 Sin(90°－x)Cos(360°+x)+Sin(－x)Cos(270°－x)=?
Sol
Sin(90°－x)=Cosx
Cos(360°+x)=Cosx
Sin(－x)=－Sinx
Cos(270°－x)=－Sinx
Sin(90°－x)Cos(360°+x)+Sin(－x)Cos(270°－x)
=CosxCosx+(－Sinx)(－Sinx)
=Cos^2 x+Sin^2 x
=1