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Algebra Difficulty 3.0 AMC 10/12 Find the answer

Let r=532+32r = \sqrt{\frac{\sqrt{53}}{2} + \frac{3}{2}}. There is a unique triple of positive integers (a,b,c)(a, b, c) such that r100=2r98+14r96+11r94r50+ar46+br44+cr40r^{100} = 2r^{98} + 14r^{96} + 11r^{94} - r^{50} + ar^{46} + br^{44} + cr^{40}. What is the value of a2+b2+c2a^{2} + b^{2} + c^{2}?

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

Solution

Suppose that r=532+32r = \sqrt{\frac{\sqrt{53}}{2} + \frac{3}{2}}. Thus, r2=532+32r^{2} = \frac{\sqrt{53}}{2} + \frac{3}{2} and so 2r2=53+32r^{2} = \sqrt{53} + 3 or 2r23=532r^{2} - 3 = \sqrt{53}. Squaring both sides again, we obtain (2r23)2=53(2r^{2} - 3)^{2} = 53 or 4r412r2+9=534r^{4} - 12r^{2} + 9 = 53 which gives 4r412r244=04r^{4} - 12r^{2} - 44 = 0 or r43r211=0r^{4} - 3r^{2} - 11 = 0 or r4=3r2+11r^{4} = 3r^{2} + 11. Suppose next that r100=2r98+14r96+11r94r50+ar46+br44+cr40r^{100} = 2r^{98} + 14r^{96} + 11r^{94} - r^{50} + ar^{46} + br^{44} + cr^{40} for some positive integers a,b,ca, b, c. Since r0r \neq 0, we can divide by r40r^{40} to obtain r60=2r58+14r56+11r54r10+ar6+br4+cr^{60} = 2r^{58} + 14r^{56} + 11r^{54} - r^{10} + ar^{6} + br^{4} + c. Now using the relationship r4=3r2+11r^{4} = 3r^{2} + 11, we can see that r602r5814r5611r54=r54(r62r414r211)=r54(r2(3r2+11)2r414r211)=r54(3r4+11r22r414r211)=r54(r43r211)=r54(0)=0r^{60} - 2r^{58} - 14r^{56} - 11r^{54} = r^{54}(r^{6} - 2r^{4} - 14r^{2} - 11) = r^{54}(r^{2}(3r^{2} + 11) - 2r^{4} - 14r^{2} - 11) = r^{54}(3r^{4} + 11r^{2} - 2r^{4} - 14r^{2} - 11) = r^{54}(r^{4} - 3r^{2} - 11) = r^{54}(0) = 0. Therefore, the equation is equivalent to the much simpler equation r10=ar6+br4+cr^{10} = ar^{6} + br^{4} + c. Next, we express r10r^{10} and r6r^{6} as combinations of r2r^{2} and constant terms. (To do this, we will need to express r8r^{8} in this way too.) r6=r2r4=r2(3r2+11)=3r4+11r2=3(3r2+11)+11r2=20r2+33r^{6} = r^{2}r^{4} = r^{2}(3r^{2} + 11) = 3r^{4} + 11r^{2} = 3(3r^{2} + 11) + 11r^{2} = 20r^{2} + 33, r8=r2r6=r2(20r2+33)=20r4+33r2=20(3r2+11)+33r2=93r2+220r^{8} = r^{2}r^{6} = r^{2}(20r^{2} + 33) = 20r^{4} + 33r^{2} = 20(3r^{2} + 11) + 33r^{2} = 93r^{2} + 220, r10=r2r8=r2(93r2+220)=93r4+220r2=93(3r2+11)+220r2=499r2+1023r^{10} = r^{2}r^{8} = r^{2}(93r^{2} + 220) = 93r^{4} + 220r^{2} = 93(3r^{2} + 11) + 220r^{2} = 499r^{2} + 1023. Therefore, the equation r10=ar6+br4+cr^{10} = ar^{6} + br^{4} + c is equivalent to 499r2+1023=a(20r2+33)+b(3r2+11)+c499r^{2} + 1023 = a(20r^{2} + 33) + b(3r^{2} + 11) + c. Rearranging, we obtain 0=r2(20a+3b499)+(33a+11b+c1023)0 = r^{2}(20a + 3b - 499) + (33a + 11b + c - 1023). Therefore, if 20a+3b=49920a + 3b = 499 and 33a+11b+c=102333a + 11b + c = 1023, then the equation is satisfied. So the original problem is equivalent to finding positive integers a,b,ca, b, c with 20a+3b=49920a + 3b = 499 and 33a+11b+c=102333a + 11b + c = 1023. We proceed by finding pairs (a,b)(a, b) of positive integers that satisfy 20a+3b=49920a + 3b = 499 and then checking to see if the value of c=102333a11bc = 1023 - 33a - 11b is positive. Since we need to find one triple (a,b,c)(a, b, c) of positive integers, we do not have to worry greatly about justifying that we have all solutions at any step. Since 20a20a has a ones digit of 0 and 20a+3b=49920a + 3b = 499, then the ones digit of 3b3b must be 9, which means that the ones digit of bb must be 3. If b=3b = 3, we obtain 20a=4993b=49020a = 499 - 3b = 490 and so aa is not an integer. If b=13b = 13, we obtain 20a=4993b=46020a = 499 - 3b = 460 and so a=23a = 23. Note that, from (a,b)=(23,13)(a, b) = (23, 13), we can obtain additional solutions by noticing that 20(3)=3(20)20(3) = 3(20) and so if we decrease aa by 3 and increase bb by 20, the sum 20a+3b20a + 3b does not change. However, it turns out that if (a,b)=(23,13)(a, b) = (23, 13), then c=102333(23)11(13)=121c = 1023 - 33(23) - 11(13) = 121. Since we are only looking for a unique triple (a,b,c)(a, b, c), then (a,b,c)=(23,13,121)(a, b, c) = (23, 13, 121). Finally, a2+b2+c2=232+132+1212=15339a^{2} + b^{2} + c^{2} = 23^{2} + 13^{2} + 121^{2} = 15339.

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