Ocaml challenge on Creating The Mathematical SImpson's rule (https://www.codewars.com/kata/565abd876ed46506d600000d/train/ocaml) .

simpsons.ml

let simpson n = 
    let f x = (3.0 /. 2.0) *. sin x *. sin x *. sin x in 
    let pi = (float_of_int 22 /. float_of_int 7) in
    let h = pi /. float_of_int n in
    let rec fstSum idx n acc = 
        if idx > n then (acc) else
        fstSum (idx+1) n (acc +. f (float_of_int ((2 * idx) -1) *. h)) in
    let rec sndSum idx n acc = 
        if idx > n then (acc) else
        sndSum (idx+1) n (acc +. f (float_of_int (2 * idx) *. h)) in
    let aux n = pi /. float_of_int (3 * n) *. ( f 0.0 +. f pi +. (4.0 *. fstSum 1 (n/2) 0.0) +. (2.0 *. sndSum 1 (n/2 - 1) 0.0)) in
    aux n

I just realized sin^3(x)!== sin (xxx)

but

sin^3(x) === [sin(x)]^3

Made necessary change, the solution now works.