Motorola M500 Manuel d'utilisateur Page 13

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M500 180 Page 11
Solution 178.7 Series
Show that
x
1 x
x
3
1 x
3
+
x
5
1 x
5
. . . =
x
1 + x
2
+
x
2
1 + x
4
+
x
3
1 + x
6
+. . . .
John Bull
This cannot be true for all x. A simple inspection shows it to be undefined
for x = 1. So we proceed on the assumption that 1 < x < 1.
Call the LHS series S, and expand each term by the binomial theorem,
which is permissible if and only if 1 < x < 1. We then have
S = x + x
2
+ x
3
+ x
4
+ . . .
x
3
x
6
x
9
x
12
. . .
+ x
5
+ x
10
+ x
15
+ x
20
. . .
x
7
x
14
x
21
x
28
. . .
+ . . . ,
where each term is expressed as a separate row. Now rearrange this so that
each column is expressed as a row, then
S = x x
3
+ x
5
x
7
+ . . .
+ x
2
x
6
+ x
10
x
14
+ . . .
+ x
3
x
9
+ x
15
x
21
+ . . .
+ x
4
x
12
+ x
20
x
28
+ . . .
+ . . . .
Each row can now be seen to be a binomial series expansion of each succes-
sive term on the RHS. Thus it is shown that LHS = RHS, as required.
Solved in a similar manner by Peter Fletcher.
Problem 180.2 Unlimited prize
Paul Richards
A casino owner devises a gambling game for which he can advertise an
unlimited prize, but which also has a predictable distribution of payouts.
A gambler pays a stake, and the croupier throws a six-sided die repeat-
edly until all the numbers from 1 to 6 inclusive have appeared, irrespective
of repetition. The gambler receives winnings proportional to the number of
throws of the dice.
What is the correct stake to make the game fair?
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