sicp-all-tasks/sicp/1_002e40

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
    
    +
    a
    
      x
      2
    
    +
    b
    x
    +
    c
  
.