Why derivative of sin x is cos x, only if x is in radians?

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Discussion Overview

The discussion centers on why the derivative of the sine function is equal to the cosine function specifically when the angle is measured in radians, exploring the implications of using different angle measures such as degrees.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants argue that the limit definition of a derivative should not be affected by whether the angle is in radians or degrees.
  • Others suggest that the radian measure is unique because it relates angles to arc lengths, making calculus work more seamlessly with radians.
  • A participant explains the application of the chain rule, noting that if the sine function is expressed in degrees, it must be converted to radians to correctly differentiate it, resulting in a derivative that includes a factor of π/180.
  • Another participant highlights that the limit \(\lim_{x→0}\frac{sin x}{x} = 1\) is valid only when \(x\) is in radians, emphasizing its importance in deriving the derivative of sine from first principles.

Areas of Agreement / Disagreement

Participants express differing views on the impact of angle measurement on the derivative of sine, with some asserting that radians are necessary for the derivative to hold true, while others question the necessity of this distinction.

Contextual Notes

The discussion does not resolve the underlying assumptions about the relationship between angle measures and derivatives, nor does it clarify the implications of using different units on the limit definitions.

Who May Find This Useful

Readers interested in calculus, particularly those exploring the foundations of trigonometric derivatives and the significance of angle measures in mathematical analysis.

supermiedos
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What differente does it make? As far as I can see it, the limit definition of a derivative shouldn't be affected by the fact that x is expressed in radians or degrees...
 
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supermiedos said:
What differente does it make? As far as I can see it, the limit definition of a derivative shouldn't be affected by the fact that x is expressed in radians or degrees...

The limit definition of the derivative relies heavily on approximations to length. The radian has 2 meanings, that of an angle and that of a length so calculus works very nicely when angles are represented in radians.

:smile:
 
You can think of it mechanically in terms of the chain rule. If the nice derivative rule holds for radians, it can't hold for an angle measure that differs from radians by a factor.
If f(y)=siny, where y is in degrees, then f(y)=sin(yπ/180), where y is in degrees but the argument of the sine is now in radians so we know how to take its derivative. This has a derivative of π/180*cos(πy/180). The logic here is the same as why, for example [itex]\frac {d}{dx} e^x = e^x[/itex] but [itex]\frac {d}{dx} e^{2x} \neq e^{2x}[/itex]
 
Last edited:
The limit [itex]\lim_{x→0}\frac{sin x}{x} = 1[/itex] holds only when x is in radians. This limit is essential to differentiating sin(x) from first principles.

BiP
 
Thank you so much for your answers, now I see it clearly :)
 

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