1.31 (另一种求圆周率的算法)

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(define (product term a next b)   (if (> a b)       1       (* (term a)          (product term (next a) next b))))(product (lambda (x) x)          1          (lambda (i) (+ i 1))          5)

　　

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(define (product term a next b)   (define (iter a result)     (if (> a b)         result         (iter (next a)               (* (term a) result))))   (iter a 1))   (define (product-normal term a next b)   (if (> a b)       1       (* (term a)          (product term (next a) next b))))   (define (factorial n)   (product (lambda (x) x)            1            (lambda (i) (+ i 1))            n))   (define (numer-term i)   (cond ((= i 1) 2)         ((even? i) (+ i 2))         (else (+ i 1))))   (define (denom-term i)   (if (odd? i)       (+ i 2)       (+ i 1)))   (define (pi n)   (* 4      (exact->inexact ; exact->inexact 函数转换除法的商,确保计算所得的结果为浮点数格式(而不是分数格式)         (/ (product numer-term                     1                     (lambda (i) (+ i 1))                     n)            (product denom-term                     1                     (lambda (i) (+ i 1))                     n)))))   (pi 50000)