Skip to main content
PATurmel12
PATurmel12
Inspiring
September 27, 2021
Answered

How to convert radians to degrees while using Math..asin

  • September 27, 2021
  • 1 reply
  • 1042 views

If


     trace(Math.sin((Math.PI/6)));          //          .499999       Correct

     trace(Math.asin((.499999)));          //          .523597        Correct

 

but


     trace(Math.sin(30*Math.PI/180));          //          0.49999        Correct

      trace(Math.asin(.49999*Math.PI/180));          //          0.008726        Wrong

 

else

 

      trace(Math.asin(.49999*180/Math.PI));          //          30.000        Correct ?

 

My Math.asin function should use the radian to degrees conversion factor of  Math.PI/180 but appears to be 180/Math.PI.

Can anyone explain why this is?

 

This topic has been closed for replies.
Correct answer kglad

the trig functions are the inverse of the arc trig functions. the trig functions have an argument that's an angle (in radians) and yield a number.  the arc functions have a number for an argument and yield an angle (in radians).

 

so yes, if you want the sin of 30 degrees, use:

 

Math.sin(30*Math.PI/180);  // converted angle to radians

 

if you want the angle (in degrees) whose sin is .5, use:

 

Math.asin(.5)*180/Math.PI

 

and while our brains are perfectly capable of working with radians without degree conversion, when it comes to digital computers, that doesn't work so well.  a digital computer isn't go to tell you asin(PI/4) = sqrt(2)/2.  and while i've seen the decimal output of that so many times, i know .707...is sqrt(2)/2, i would not recognize the radian equivalent of 35 degrees.  (and i have a phd in math.)

1 reply

kglad
kglad
Community Expert
September 27, 2021

to convert rad to deg, multiply by 180/Math.PI

 

Math.asin(.5)*180/Math.PI;  // should be 30 (degrees)

 

i don't know if you have typos in this forum (ie, your parenthesis are misplaced), but some of what you posted (adjusting the argument in asin) is non-sense from a mathematical point of view.

PATurmel12
PATurmel12Author
Inspiring
September 27, 2021

Thanks for the reply, it is interesting the code I posted has always worked for sin, cos, and tan, but maybe there's something funny about the parenthesis as you suggest.

If anyone could elaborate on this, I don't see exactly what is wrong.

 

 

kglad
kgladCorrect answer
Community Expert
September 27, 2021

the trig functions are the inverse of the arc trig functions. the trig functions have an argument that's an angle (in radians) and yield a number.  the arc functions have a number for an argument and yield an angle (in radians).

 

so yes, if you want the sin of 30 degrees, use:

 

Math.sin(30*Math.PI/180);  // converted angle to radians

 

if you want the angle (in degrees) whose sin is .5, use:

 

Math.asin(.5)*180/Math.PI

 

and while our brains are perfectly capable of working with radians without degree conversion, when it comes to digital computers, that doesn't work so well.  a digital computer isn't go to tell you asin(PI/4) = sqrt(2)/2.  and while i've seen the decimal output of that so many times, i know .707...is sqrt(2)/2, i would not recognize the radian equivalent of 35 degrees.  (and i have a phd in math.)

Terms & ConditionsAccessibility statement
Using the community Terms of use Privacy Cookie preferences Do not sell or share my personal information ad choicesAdChoices