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I find that when I calculate $\cos{1}$ in ...

  • Google: $0.540$
  • Ubuntu: $0.998$

I believe its different units. But I wonder which should I be using? The question I am doing is use bisection method to find $\cos{x} = x$ when $x \in [0,1]$. For now, I am using Google as its close to what I get from a Graphing Utility

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Google is using radians and Ubuntu is using degrees. Which one you want to use depends on where you got the question from, but I assume radians were intended. – Qiaochu Yuan Feb 26 '12 at 13:59
Google is using radians; Ubuntu is using degrees. I would stick with radians. – The Chaz 2.0 Feb 26 '12 at 14:01
Grownups use radians. – Gerry Myerson Feb 27 '12 at 0:03
up vote 2 down vote accepted

As Qiaochu and The Chaz say in the comments, the difference is because Google is showing you the cosine of 1 radian, while Ubuntu is showing you the cosine of 1 degree.

As The Chaz said, I suggest you stick to radians, in particular if you are doing anything related to calculus. The assertion that $(\sin x)'=\cos x$, $(\cos x)'=-\sin x$ is not true if $x$ is in degrees.

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