题目：(define (add-interval x y)(make-interval (+ (lower-bound x) (lower-bound y))(+ (upper-bound x) (upper-bound y))))和的最小值应该是两个区间下界之和，和的最大值应该是两个区间上界之和
求出:
1.upper-bound 和 lower-bound的定义
2.定义 sub-interval
这是我的理解图
(define (make-interval a b) (cons a b))
所以不难看出
(define (lower-bound interval) (car interval) )
interval 可以是 x 或 y
同理：
(define (upper-bound interval) (cdr interval) )
2.8 定义 sub-interval
(define (sub-interval x y)(make-interval (- (lower-bound x) (upper-bound y))(- (upper-bound x) (lower-bound y))))