(define (accumulate combiner null-value  term a next b)
  (if (> a b)
      null-value
      (combiner (term a)
                (accumulate combiner null-value term (next a) next b))))
(define (sum term a next b)
  (accumulate + 0 term a next b))
(define (product term a next b)
  (accumulate * 1 term a next b))
(define (accumulate-iter combiner null-value term a next b)
  (define (iter a result)
    (if (> a b)
        result
        (iter (next a) (combiner (term a) result))))
  (iter a null-value))
(define (sum-iter term a next b)
  (accumulate-iter + 0 term a next b))
(define (product-iter term a next b)
  (accumulate-iter * 1 term a next b))