More about Trigonometry

Since cos ( - x) = cos x
so cos [ - (360 - x)] = cos (x - 360) = cos (360 - x).

LHS:
cos(θ-360) = cos θ
We need to draw a coordinate plane,
no matter θ-360, -720, -1080, etc.
they all on exactly the same line of the coordinate plane, therefore have same value.

RHS:
cos(360-θ) = cos (-θ)
i.e., drawing a line clockwise counting from the positive portion of the x-axis.

Since cos in Quadrant I and IV both have positive value (rule of CAST).
Therefore cos(θ) = cos(-θ)
and so, cos(θ-360)=cos(360-θ)



若覺得我幫到你，請到我的 Facebook Page 比個 Like 我，感激！
經數學人
http://fb.com/ronaldchik