sicp exercise 1.40

;  Exercise 1.40.  Define a procedure cubic that can be used together with the newtons-method procedure
;  in expressions of the form

;  (newtons-method (cubic a b c) 1)

;  to approximate zeros of the cubic x^3 + ax^2 + bx + c

(define (cube x) (* x x x))
(define (square x) (* x x))

(define (cubic a b c)
  (lambda (x)
    (+ (cube x)
       (* a (square x))
       (* b x)
       c)))