Can't calculate the area of regular polygon - wrong result using the tangent formula

I have this method for calculating regular polygon area:

    public double getArea()  {
        return (sideLength *
            sideLength * sides) /
            (4 * Math.tan(180 / (double) sides));
    }

for sideLength and sides both being equal to 10 it returns -219.816218.
"
However this online calculator: https://www.omnicalculator.com/math/regular-polygon-area
returns 769.4. What is wrong with my method? The formula I use is specified here
.

Use the following return statement

return (sideLength  * sideLength  * sides) / (4 * Math.tan((180 / sides) * 3.14159 / 180));

Here, *(3.14159 / 180) is added to convert the area from degree converted to radians

The arguments to trigonometric functions are defined on radians, not degrees. Use Math.toRadians to convert the angle in degrees to radians - like this:

Math.tan(Math.toRadians(180 / (double) sides))

Or do this computation in radians to start with.

Math.tan(Math.PI / sides)

The problem is that the Math.tan function uses radians as it's default unit of measure. Use this instead:

(4*Math.tan(Math.PI/180 * 180/(double) sides))