Cosine of Obtuse Angle is Negative

Theorem

Let $\theta$ be an obtuse angle.

Then:

$\cos \theta < 0$

where $\cos$ denotes the cosine function.


Proof


This theorem requires a proof.
In particular: trivial but I don't want to be distracted.
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Sources

  • 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): trigonometric function
  • 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): trigonometric function
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