•  At the beginning the calculation is the following:
5 / (3 + pow( 1 - sqrt(x*x+y*y), 2) )  with  x = -1/2; y = sqrt(3)/2
(that is:   5 / ( 3 + ( 1 − √(x²+y²) ) 2 )
The output is: 
1.6666666666666667
You can change the calculation however you want.

The original file:

<head><title>calculation</title><script> with(Math) { 

x = -1/2; y = sqrt(3)/2
u = 5 / (3 + pow( 1 - sqrt(x*x+y*y), 2) )

document.write(u)

}

//  abs(a)     // the absolute value of a
//  acos(a)    // arc cosine of a
//  asin(a)    // arc sine of a
//  atan(a)    // arc tangent of a
//  atan2(a,b) // arc tangent of a/b
//  ceil(a)    // integer closest to a and not less than a
//  cos(a)     // cosine of a
//  exp(a)     // exponential of a
//  floor(a)   // integer closest to and not greater than a
//  log(a)     // log of a base e
//  max(a,b)   // the maximum of a and b
//  min(a,b)   // the minimum of a and b
//  pow(a,b)   // a to the power b
//  random()   // pseudorandom number in the range 0 to 1
//  round(a)   // integer closest to a 
//  sin(a)     // sine of a
//  sqrt(a)    // square root of a
//  tan(a)     // tangent of a
//  PI         // π
//  exp(1)     // e

</script>
</head>

•  If I want calculate:

      I change:
x = -1/2; y = sqrt(3)/2
u = 5 / (3 + pow( 1 - sqrt(x*x+y*y), 2) )
document.writeln(u)

      with:
x = 5/6; y = -1/3; z = 0.5
u = pow(x+y,2)/z
document.write(u)

      I have:
0.5

•  With:

x = 3; a = x*x; b = a+3; c = x+5; d = b/c; u = sqrt(d)
document.write("d = " + d); document.write(" | u = " + u)

      I have:

d = 1.5 | u = 1.224744871391589

      If I want to round u to the 3rd digit after ".":

x = 3; a = x*x; b = a+3; c = x+5; d = b/c; u = sqrt(d)
document.write("d = " + d); document.write(" | u = " + round(u*1000)/1000 )

      I have:

d = 1.5 | u = 1.225