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 x3 + ax2 + bx + c.

 
(define (cubic a b c)
  (lambda (x)
    (+  (cube x)
        (* a (square x))
        (* b x)
        c)))
 
 
 
(define (cube x) (* x x x))
 
(newtons-method (cubic a b c) 1)