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1) Write a higher-order procedure ADD-EXCEPTION that takes three
arguments: two numbers (N1 and N2) and a procedure (FN). ADD-EXCEPTION
returns a procedure which behaves exactly like FN except when the
argument is equal to N1 in which case it returns N2. As an example
consider the procedure USUALLY-SQUARE below:

> (define (square x) (* x x))

> (define usually-square
     (add-exception 42 0 square))

> (usually-square 5)
25

> (usually-square 0)
0 

> (usually-square 40)
1600

> (usually-square 42
0

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2) Is there any difference between the following definitions of PI?
Explain your answer.

(define pi (/ 22 7))
(define pi (lambda () (/ 22 7)))
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3) What are the values of the folliwng expressions?

1) ((f g) 0)
2) ((f (f g)) 0)

where f and g are defined as follows:

(define (f h) (lambda (x) (h (h (h x)))))
(define (g x) (+ 1 x))

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4) Write a function that returns the result of converting a temperture i
Fahrenheit to its equivalent in Celsius. Use the formula:

celsius = (fahrenheit - 32) * 5/9

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5) Write a procedure named PERFECT? that tests whether a given integer is
a perfect number. Like this:

> (perfect? 5)
#f

> (perfect? 6)
#t

> (perfect? 28)
#t

Perfect numbers are numbers that are equal to the sum of their
devisors. Note, for example, that 6=1+2+3 and 28=1+2+4+7+14. Another
interesting fact that one may exploit in this problem is that a
devisor of a number N is at most N/2.

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6) Assume the following definitions:

(define x 10)
(define y 20)

What are the values of the following expressions:

(let ((x 11))
   (+ x y))

(let ((x y)
      (y x))
   (+ x y))

(let ((x y))
  (let ((y x))
     (+ x y)))
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