Maths Olympiad Prep

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Algebra Difficulty 6.3 National olympiad Find the answer

Which number is greater:
A=2.00041.00042+2.0004, or B=2.00021.00022+2.0002,A=\frac{2.00\ldots04}{1.00\ldots04^2+2.00\ldots04},\text{ or }B=\frac{2.00\ldots02}{1.00\ldots02^2+2.00\ldots02},where each of the numbers above contains 19981998 zeros?

This was a multiple-choice question, but the options didn't survive into the source we have. The answer given is B, and the solution below works it through.

Solution

1. Let x=2.0004 x = 2.00\ldots04 and y=2.0002 y = 2.00\ldots02 . Note that x>y x > y since x x has more trailing zeros before the final digit 4 compared to y y which has more trailing zeros before the final digit 2.
2. We need to compare A=x(x1)2+x A = \frac{x}{(x-1)^2 + x} and B=y(y1)2+y B = \frac{y}{(y-1)^2 + y} .
3. Simplify the expressions for A A and B B :
A=xx22x+1+x=xx2x+1 A = \frac{x}{x^2 - 2x + 1 + x} = \frac{x}{x^2 - x + 1}
B=yy22y+1+y=yy2y+1 B = \frac{y}{y^2 - 2y + 1 + y} = \frac{y}{y^2 - y + 1}
4. We claim that xx2x+1<yy2y+1 \frac{x}{x^2 - x + 1} < \frac{y}{y^2 - y + 1} .
5. To compare these fractions, we can cross-multiply to obtain:
x(y2y+1)<y(x2x+1) x(y^2 - y + 1) < y(x^2 - x + 1)
6. Expanding both sides:
xy2xy+x<yx2yx+y xy^2 - xy + x < yx^2 - yx + y
7. Rearrange the terms:
xy2yx2<yx xy^2 - yx^2 < y - x
8. Factor out common terms:
xy(yx)<yx xy(y - x) < y - x
9. Since yx<0 y - x < 0 (because x>y x > y ), we can divide both sides by yx y - x (noting that dividing by a negative number reverses the inequality):
xy>1 xy > 1
10. Given that x x and y y are very large numbers (each containing 1998 zeros), xy xy is indeed much greater than 1. Therefore, the inequality xy>1 xy > 1 holds true.

Thus, we have shown that:
xx2x+1<yy2y+1 \frac{x}{x^2 - x + 1} < \frac{y}{y^2 - y + 1}

The final answer is B \boxed{ B }

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.