AlgebraDifficulty 6.3National olympiadFind the answer
Which number is greater: A=1.00…042+2.00…042.00…04, or B=1.00…022+2.00…022.00…02,where each of the numbers above contains 1998 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=2.00…04 and y=2.00…02. Note that x>y since x has more trailing zeros before the final digit 4 compared to y which has more trailing zeros before the final digit 2. 2. We need to compare A=(x−1)2+xx and B=(y−1)2+yy. 3. Simplify the expressions for A and B: A=x2−2x+1+xx=x2−x+1x B=y2−2y+1+yy=y2−y+1y 4. We claim that x2−x+1x<y2−y+1y. 5. To compare these fractions, we can cross-multiply to obtain: x(y2−y+1)<y(x2−x+1) 6. Expanding both sides: xy2−xy+x<yx2−yx+y 7. Rearrange the terms: xy2−yx2<y−x 8. Factor out common terms: xy(y−x)<y−x 9. Since y−x<0 (because x>y), we can divide both sides by y−x (noting that dividing by a negative number reverses the inequality): xy>1 10. Given that x and y are very large numbers (each containing 1998 zeros), xy is indeed much greater than 1. Therefore, the inequality xy>1 holds true.
Thus, we have shown that: x2−x+1x<y2−y+1y
The final answer is B
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Source: NuminaMath-1.5,
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