Exercise 1.21. Use the smallest-divisor procedure to find the smallest divisor of each of the following numbers: 199, 1999, 19999.
直接求值就可以：
(define (smallest-divisor n)
(find-divisor n 2))
(define (find-divisor n test-divisor)
(cond ((> (square test-divisor) n) n)
((divides? test-divisor n) test-divisor)
(else (find-divisor n (+ test-divisor 1)))))
(define (divides? a b)
(= (remainder b a) 0))
(smallest-divisor 199) ; => 199
(smallest-divisor 1999) ; => 1999
(smallest-divisor 19999) ; => 7