SICP 练习1.19

经过推导

p′=p2+q2

q′=2pq+q2

(define (fib n)  (fib-iter 1 0 0 1 n))(define (fib-iter a b p q count)  (cond ((= count 0) b)        ((even? count)         (fib-iter a                   b                   (+ (* q q) (* p p))                   (+ (* 2 p q) (* q q))                   (/ count 2)))        (else (fib-iter (+ (* b q) (* a q) (* a p))                        (+ (* b p) (* a q))                        p                         q                        (- count 1)))))(fib 10)