Let r=253+23. There is a unique triple of positive integers (a,b,c) such that r100=2r98+14r96+11r94−r50+ar46+br44+cr40. What is the value of a2+b2+c2?
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Solution
Suppose that r=253+23. Thus, r2=253+23 and so 2r2=53+3 or 2r2−3=53. Squaring both sides again, we obtain (2r2−3)2=53 or 4r4−12r2+9=53 which gives 4r4−12r2−44=0 or r4−3r2−11=0 or r4=3r2+11. Suppose next that r100=2r98+14r96+11r94−r50+ar46+br44+cr40 for some positive integers a,b,c. Since r=0, we can divide by r40 to obtain r60=2r58+14r56+11r54−r10+ar6+br4+c. Now using the relationship r4=3r2+11, we can see that r60−2r58−14r56−11r54=r54(r6−2r4−14r2−11)=r54(r2(3r2+11)−2r4−14r2−11)=r54(3r4+11r2−2r4−14r2−11)=r54(r4−3r2−11)=r54(0)=0. Therefore, the equation is equivalent to the much simpler equation r10=ar6+br4+c. Next, we express r10 and r6 as combinations of r2 and constant terms. (To do this, we will need to express r8 in this way too.) r6=r2r4=r2(3r2+11)=3r4+11r2=3(3r2+11)+11r2=20r2+33, r8=r2r6=r2(20r2+33)=20r4+33r2=20(3r2+11)+33r2=93r2+220, r10=r2r8=r2(93r2+220)=93r4+220r2=93(3r2+11)+220r2=499r2+1023. Therefore, the equation r10=ar6+br4+c is equivalent to 499r2+1023=a(20r2+33)+b(3r2+11)+c. Rearranging, we obtain 0=r2(20a+3b−499)+(33a+11b+c−1023). Therefore, if 20a+3b=499 and 33a+11b+c=1023, then the equation is satisfied. So the original problem is equivalent to finding positive integers a,b,c with 20a+3b=499 and 33a+11b+c=1023. We proceed by finding pairs (a,b) of positive integers that satisfy 20a+3b=499 and then checking to see if the value of c=1023−33a−11b is positive. Since we need to find one triple (a,b,c) of positive integers, we do not have to worry greatly about justifying that we have all solutions at any step. Since 20a has a ones digit of 0 and 20a+3b=499, then the ones digit of 3b must be 9, which means that the ones digit of b must be 3. If b=3, we obtain 20a=499−3b=490 and so a is not an integer. If b=13, we obtain 20a=499−3b=460 and so a=23. Note that, from (a,b)=(23,13), we can obtain additional solutions by noticing that 20(3)=3(20) and so if we decrease a by 3 and increase b by 20, the sum 20a+3b does not change. However, it turns out that if (a,b)=(23,13), then c=1023−33(23)−11(13)=121. Since we are only looking for a unique triple (a,b,c), then (a,b,c)=(23,13,121). Finally, a2+b2+c2=232+132+1212=15339.
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