SOLVED Hp Prime - CAS inconsistent derivatives of sin, cos, tan

[CR Haeger]

In CAS I notice now that taking first derivative of sin, cos or tan yields different results if in radians versus degrees mode.

In radians mode, d sin(x)/dx gives cos(x)
In degrees mode it gives PI * cos(x)/180

I assume these should be consistent and match the result from radians mode?

Running 6030 firmware.

[cdecastro]

This is correct. The derivative rules d(sin(x))=cos(x) etc.. only hold when the angle is measured in radians. If the angle is in degrees the appropriate rule is found by applying the chain rule.

i.e. if the given angle x is measured in radians as x_rad and in degrees as x_deg, then x_deg = 180/Pi * x_rad, so

d( sin( Pi/180 * x_deg ) ) = cos( Pi/180 * x_deg ) * Pi/180 (using chain rule)
= cos( Pi/180 * x_deg) * Pi/180

Regards,
Chris

[Tugdual]

(05-29-2014 10:40 AM)cdecastro Wrote:  This is correct. The derivative rules d(sin(x))=cos(x) etc.. only hold when the angle is measured in radians. If the angle is in degrees the appropriate rule is found by applying the chain rule.

i.e. if the given angle x is measured in radians as x_rad and in degrees as x_deg, then x_deg = 180/Pi * x_rad, so

d( sin( Pi/180 * x_deg ) ) = cos( Pi/180 * x_deg ) * Pi/180 (using chain rule)
= cos( Pi/180 * x_deg) * Pi/180

Regards,
Chris

Correct
$$(g\circ f)'=g'\circ f*f'\\ \sin { (a*x)'=cos(a*x)*a } $$
While converting from degree to radian you get the conversion factor
$$a=\frac { \pi }{ 180 } $$

[CR Haeger]

Yes, of course you and the Prime are both correct...

I got tripped up as I used to do DERVX in CAS with the 50G and it forced me to switch from deg --> rad. The Prime provides correct answers in either mode. This would seem to be a great feature for Geometry teachers.

Thanks again.

ps TW: 3, Me: 0

[Tim Wessman]

I would think it is more 3 to <some much larger number> and not 0...

[CR Haeger]

(05-29-2014 01:30 PM)Tim Wessman Wrote:  I would think it is more 3 to <some much larger number> and not 0...

Nope - 3-0 (since the new firmware release). However, Im still waiting to hear if there is a way to recover my custom spreadsheet apps content following the upgrade. Id prefer not to retype all this back in.

[Michael de Estrada]

(05-29-2014 01:53 PM)CR Haeger Wrote:  

(05-29-2014 01:30 PM)Tim Wessman Wrote:  I would think it is more 3 to <some much larger number> and not 0...

However, I'm still waiting to hear if there is a way to recover my custom spreadsheet apps content following the upgrade. Id prefer not to retype all this back in.

Start typing.

[rprosperi]

(05-29-2014 01:53 PM)CR Haeger Wrote:  ...Id prefer not to retype all this back in.

(05-29-2014 02:11 PM)Michael de Estrada Wrote:  Start typing.

Yup! What he said... It got me too.