SICP 练习1.7

这个是改进good-enough?函数，容易想到，guess (improve guess x) 分别是上一次迭代和下一次迭代的结果，那么就可以测量这个变化来改进good-enough?函数

(define (sqrt-iter guess x)  (if (good-enough? guess (improve guess x))    guess    (sqrt-iter (improve guess x) x)))(define (improve guess x)  (average guess (/ x guess)))(define (average x y)  (/ (+ x y) 2))(define (good-enough? guess x)  (< (/ (abs (- guess x)) guess) 0.00001))(define (square x) (* x x))(define (sqrt x)  (sqrt-iter 1.0 x))(sqrt 9)(sqrt (+ 100 37))(sqrt (+ (sqrt 2) (sqrt 3)))(square (sqrt 1000))