Maths Olympiad Prep

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Algebra Difficulty 3.2 AMC 10/12 Find the answer

How many positive integers nn with n100n \leq 100 can be expressed as the sum of four or more consecutive positive integers?

A number or a short expression. Spacing and $ signs are ignored.

Solution

We consider first the integers that can be expressed as the sum of exactly 4 consecutive positive integers. The smallest such integer is 1+2+3+4=101+2+3+4=10. The next smallest such integer is 2+3+4+5=142+3+4+5=14. We note that when we move from k+(k+1)+(k+2)+(k+3)k+(k+1)+(k+2)+(k+3) to (k+1)+(k+2)+(k+3)+(k+4)(k+1)+(k+2)+(k+3)+(k+4), we add 4 to the total (this equals the difference between k+4k+4 and kk since the other three terms do not change). Therefore, the positive integers that can be expressed as the sum of exactly 4 consecutive positive integers are those integers in the arithmetic sequence with first term 10 and common difference 4. Since n100n \leq 100, these integers are 10,14,18,22,26,30,34,38,42,46,50,54,58,62,66,70,74,78,82,86,90,94,9810,14,18,22,26,30,34,38,42,46,50,54,58,62,66,70,74,78,82,86,90,94,98. There are 23 such integers. Next, we consider the positive integers n100n \leq 100 that can be expressed as the sum of exactly 5 consecutive positive integers. The smallest such integer is 1+2+3+4+5=151+2+3+4+5=15 and the next is 2+3+4+5+6=202+3+4+5+6=20. Using an argument similar to that from above, these integers form an arithmetic sequence with first term 15 and common difference 5. Since n100n \leq 100, these integers are 15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,10015,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,100. When we exclude the integers already listed above (30, 50, 70, 90), we obtain 15,20,25,35,40,45,55,60,65,75,80,85,95,10015,20,25,35,40,45,55,60,65,75,80,85,95,100. There are 14 such integers. Next, we consider the positive integers n100n \leq 100 that can be expressed as the sum of exactly 6 consecutive positive integers. These integers form an arithmetic sequence with first term 21 and common difference 6. Since n100n \leq 100, these integers are 21,27,33,39,45,51,57,63,69,75,81,87,93,9921,27,33,39,45,51,57,63,69,75,81,87,93,99. When we exclude the integers already listed above (45,75)(45,75), we obtain 21,27,33,39,51,57,63,69,81,87,93,9921,27,33,39,51,57,63,69,81,87,93,99. There are 12 such integers. Since 1+2+3+4+5+6+7+8+9+10+11+12+13+14=1051+2+3+4+5+6+7+8+9+10+11+12+13+14=105 and this is the smallest integer that can be expressed as the sum of 14 consecutive positive integers, then no n100n \leq 100 is the sum of 14 or more consecutive positive integers. (Any sum of 15 or more consecutive positive integers will be larger than 105.) Therefore, if an integer n100n \leq 100 can be expressed as the sum of s4s \geq 4 consecutive integers, then s13s \leq 13. We make a table to enumerate the n100n \leq 100 that come from values of ss with 7s137 \leq s \leq 13 that we have not yet counted: ss & Smallest nn & Possible n100n \leq 100 & New nn \\ 7 & 28 & 28,35,42,49,56,63,70,77,84,91,9828,35,42,49,56,63,70,77,84,91,98 & 28,49,56,77,84,9128,49,56,77,84,91 \\ 8 & 36 & 36,44,52,60,68,76,84,92,10036,44,52,60,68,76,84,92,100 & 36,44,52,68,76,9236,44,52,68,76,92 \\ 9 & 45 & 45,54,63,72,81,90,9945,54,63,72,81,90,99 & 72 \\ 10 & 55 & 55,65,75,85,9555,65,75,85,95 & None \\ 11 & 66 & 66,77,88,9966,77,88,99 & 88 \\ 12 & 78 & 78,90 & None \\ 13 & 91 & 91 & None. In total, there are 23+14+12+6+6+1+1=6323+14+12+6+6+1+1=63 such nn.

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