Quadratic equation in OCaml

let square x = x *. x;;

let delta  a b c = ( square b-. (4. *. a *. c ));;

let   solve a b c   =
    if a=0. 
        then Printf.printf "Not a quadratic equation\n"
            else
                if   delta a b c  >= 0.
                    then
                        let   x1=( -.b +. sqrt(delta a b c )) /. (2. *. a)
                            and x2=  (-.b -.  sqrt(delta a b c ))  /. (2. *. a)  
                        in 
                            Printf.printf "x1 =%.5f x2=%.5f \n"  x1  x2
                    else 
                        let   x= (-.b   /. (2. *. a))
                            and i=     sqrt(4. *. a *. c -. square b)  /. (2. *. a) 
                        in Printf.printf "x+ =%.5f+i%.5f  x-=%.5f-i%.5f \n"  x  i  x i
    ;;



let () =
solve (float_of_string Sys.argv.(1))( float_of_string Sys.argv.(2) )(float_of_string Sys.argv.(3))