Monday's Solutions
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Exercise 1:

(define usable-page
  (lambda (m)
    (* (- 8.5 m m) (- 11 m m))))

Exercise 2:

plan: use divisible? from before

(define remainder
  (lambda (x y)
    (if (< x y)
        x
        (remainder (- x y) y))))

(define divisible?
  (lambda (x y)
    (= (remainder x y) 0)))

(define smallest-factor
  (lambda (n f)
    (if (>= f n)      ;No factors larger than n (there's a better bound)
        #f
        (if (divisible? n f)
            f
            (smallest-factor n (+ f 1))))))

Exercise 3:

(define prime?
  (lambda (n)
    (not (smallest-factor n 2))))