help with a formula

[MMDE]

ey, I'm kind of stuck atm...

 

$x=variable;

$y=($x*pow(2*$x,2))+($x*50);

$x=formula;

echo $x;

should output the variable

 

so anyone who can help me with the formula?

[MMDE]

I changed my mind...

this instead:

2*($x)*($x+2.5)*($x+10);

 

$x=variable;

$y=2*($x)*($x+2.5)*($x+10);

$x=formula using $y to calculate unknown $x;

echo $x;

$x should be the same as what I first assigned it to be.

[requinix]

You see the curves there? It means that for a given $y there could be three different $x values. What about that?

[MMDE]

You see the curves there? It means that for a given $y there could be three different $x values. What about that?

I know, but are you telling me I can't do what I'm trying to?

I've got help with workarounds for 2nd degree before, would it be better if I skipped the 3rd degree and just had 4th, 2nd and 1st? =o

[MMDE]

Forgot to say, and might be very important, though very obvious...

the variable is always going to be positive...

and if you look at the numbers, you will see all the nulls are negative except for one, which is 0.

[MMDE]

$x=5; // can be any positive number
echo 'x='.$x.'<br />';
$y=2*($x)*($x+2.5)*($x+10); // 2x^3 + 25x^2 + 50x
echo 'y='.$y.'<br />';
$a=2;
$b=25;
$c=50;
$d=-$y; // 2x^3 + 25x^2 + 50x + 0 - $y = 0
$x=pow((((-pow($b,3))/(27*pow($a,3)))+(($b*$c)/(6*pow($a,2)))-($d/(2*$a)))+sqrt(pow(((-pow($b,3))/(27*pow($a,3)))+(($b*$c)/(6*pow($a,2)))-($d/(2*$a)),2)+pow(($c/(3*$a))-((pow($b,2))/(9*pow($a,2))),3)),1/3)+pow((((-pow($b,3))/(27*pow($a,3)))+(($b*$c)/(6*pow($a,2)))-($d/(2*$a)))-sqrt(pow(((-pow($b,3))/(27*pow($a,3)))+(($b*$c)/(6*pow($a,2)))-($d/(2*$a)),2)+pow(($c/(3*$a))-((pow($b,2))/(9*pow($a,2))),3)),1/3)-($b/(3*$a));
echo 'x='.$x; // should echo the same as what you first assigned $x to be

 

Took me some time to write the cubic formula, but it was worth it, it worked!