【SICP练习】84 练习2.56

这道题的代码略长啊。不过我也是因此而知道Edwin上的代码居然可以复制到word上，以前还想当然的以为不能复制的，毕竟在Edwin上粘贴用的Ctrl+Y，而不是Ctrl+V。在这里我就只将书上没有的代码贴出来了。

(define (deriv exp var) (cond ((number? exp) 0) ((variable? exp) (if (same-variable? exp var) 1 0)) ((sum? exp) (make-sum (deriv (addend exp) var) (deriv (augend exp) var))) ((product? exp) (make-sum (make-product (multiplier exp) (deriv (multiplicand exp) var)) (make-product (deriv (multiplier exp) var) (multiiplicand exp)))) ((exponentiation? exp) (let ((n (exponent exp)) (u (base exp))) (make-product n (make-product (make-exponentiation u (- u 1)) (deriv u var))))) (else (error "unknown expression type -- DERIV" exp))))
(define (make-exponentiation base exponent) (cond ((= exponent 0) 1) ((= exponent 1) base) (else (list '** base exponent))))
(define (exponentiation? x) (and (pair? x) (eq? (car x) '**)))
(define (base exp) (cadr exp))
(define (exponent exp) (caddr exp))

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