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

[PATurmel12]

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?

[kglad]

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]

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 · Accepted solution]

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.)