Here’s a diagram that very visibly shows the error: <https://www.wolframalpha.com/input?i=plot+y+%3D+sin%28x%29%2C+y+%3D+x+*+%281+%E2%88%92+x%C2%B2%2F6+*+%281+%

Here’s a diagram that very visibly shows the error: <https://www.wolframalpha.com/input?i=plot+y+%3D+sin%28x%29%2C+y+%3D+x+*+%281+%E2%88%92+x%C2%B2%2F6+*+%281+%E2%88%92+x%C2%B2%2F20+*+%281+-+x%C2%B2%2F42%29%29%29+from+x%3D%E2%88%92%CF%80+to+%CF%80 https://www.wolframalpha.com/input?i=plot+y+%3D+sin%28x%29%2...>. And that’s even without accumulated float error if you start outside the range [−π, π] (huge numbers will just be wildly wrong).