; happy new year

(define (tri-list n)
  (let loop ((i 0) (ts (list 0)))
    (if (= n i) (reverse ts)
      (loop (+ i 1) (cons (+ i (car ts) 1) ts)))))

(display (tri-list 63)) (newline)

(define (tri n) (* n (+ n 1) 1/2))

(display (tri 63)) (newline)