8. Develop a proof by contradiction in the form of (1.6) of the formula in exercise 6.

(Hint: The formula is apparently true for n ¼ 0; 1; 2, and other values of n. Let N be the first integer for which it is false. From the truth for n ¼ N  1, and falsity for n ¼ N, conclude that 2N 02N and recall the remark after (1.6).)