[SOLVED] Convert cube root function into PHP

[chrischen]

Hi I need to write a PHP function for this equation:

 

[ ((2/3)(x-50))^(1/3) ] + 4

 

The problem is that PHP converts the 1/3 to a decimal or something, but I need it to take the cube root. Right now with that equation above in PHP any X value under 50 produces a nonreal answer and thus NaN as a result. The equation works fine on my Ti-84.

 

So basically I need to figure out how to take cube roots in PHP.

[AndyB]

http://ca.php.net/manual/en/function.pow.php - does that help?

[chrischen]

I tried using pow($base, 1/3) however any negative number still becomes nonreal. So it's not taking the cube root.

[chrischen]

$ret = exp( (1/3) * log((2/3)*($x-50)) ) + 4;

 

I used that but same thing, negative numbers are still not cube rooted. It returns NAN if X < 50.

[AndyB]

If I understand, the only problem now is when x<50.  Why not test x-50 to decide whether the cube root of the absolute value of x-50 needs to be multiplied by -1 or not?

[chrischen]

Acutally I solved it by using this algorithm to get the cube root. Works with negatives too.

 

function cube( $c ) {

$x = 2;

do {

$x = $x - (pow($x,3) - $c)/(3*pow($x,2));

 

$count++;

} while($count < 50);

return $x;

}

[chrischen]

Ignore this

[chrischen]

For some reason if you try to cube root -16 it breaks...

 

So I changed $x to equal 3 instead of two. All number from -99 to 99 have been tested.

 

function cube( $c ) {

$x = 3;

 

do {

$x = $x - (pow($x,3) - $c)/(3*pow($x,2));

 

$count++;

} while($count < 50);

return $x;

 

}