public function derived(Number $x, Number $limit = null): Number
    {
        $limit = $limit ?? Tangent::limit();

        return divide(
            subtract(
                $this(add($x, $limit)),
                $this($x)
            ),
            $limit
        );
    }
public function derived(Number $x, Number $limit = null): Number
    {
        $limit = $limit ?? Tangent::limit();

        return divide(
            subtract(
                $this(add($x, $limit)),
                $this($x)
            ),
            $limit
        );
    }
public function derived ( Number $ x , Number $ limit = null ) : Number { $ limit = $ limit ?? Tangent :: limit ( ) ; return divide ( subtract ( $ this ( add ( $ x , $ limit ) ) , $ this ( $ x ) ) , $ limit ) ; }
Compute the derived number of x

@param Number $x
@param Number|null $limit Value that tend to 0 (default to 0.000000000001)

@return Number
Compute the derived number of x