(altsum
) One way to see if a number is divisible by 11 is to find
the alternating sum (+ then -) of the digits of the number,
beginning at the ones digit, and see if that number is divisible
by 11. The alternating sum of the digits if 1495 is 5-9+4-1 = -1, so 1495
is not divisible by 11. Design and test the function
altsum: number(n) -> number
.
(check-expect (altsum 95) (+ 5 -9))
(check-expect (altsum 1495) (+ 5 -9 4 -1))
(sum-octa
) The n-th octagonal number is the number of dots that it
takes to draw all of the edges of a regular octagon with n dots
per edge. The formula for the n-th octagonal number is $3n^2-2n$,
or (- (* 3 n n) (* 2 n))
in Racket.
Write a function sum-octa: number(start) number(end) -> number
to find the sum of all of n-th octagonal numbers beginning with
n=start
and ending with n=end
.
(check-expect (sum-octa 1 2) 9)
(check-expect (sum-octa 2 5) 134)
n | octagonal number n |
---|---|
1 | 1 |
2 | 8 |
3 | 21 |
4 | 40 |
5 | 65 |