SICP 2.13.part1

(define (make-interval a b) (cons a b))
(define (lower-bound i) (car i))
(define (upper-bound i) (cdr i))

(define (mul-interval-small x y)
  (let ((a (lower-bound x))
        (b (upper-bound x))
        (c (lower-bound y))
        (d (upper-bound y))
        (width-x (/ (- (upper-bound x) (lower-bound x)) 2))
        (width-y (/ (- (upper-bound y) (lower-bound y)) 2)))
  (make-interval (* a c) 
                 (+ (* a c) (* (+ (* width-x c) (* width-y a)) 2)))))