Number theoryDifficulty 6.0AIME, harderFind the answer
Find the sum of all values of a+b, where (a,b) is an ordered pair of positive integers and a2+2017−b2 is a perfect square.
A number or a short expression. Spacing and $ signs are ignored.
Solution
1. We start with the equation a2+2017−b2=k2 where k is an integer. For this to be true, 2017−b2 must also be an integer. Let 2017−b2=m where m is an integer. Therefore, we have: 2017−b2=m2 This can be rewritten as: 2017=b2+m2 This implies that b2+m2=2017.
2. We need to find pairs (b,m) such that both b and m are integers. We test values of b to find corresponding m values that satisfy the equation.
3. Testing b=44: 2017−442=2017−1936=81 81=9 So, one pair is (b,m)=(44,9).
4. Testing b=9: 2017−92=2017−81=1936 1936=44 So, another pair is (b,m)=(9,44).
5. Now, we have two pairs (b,m): (44,9) and (9,44). We need to find corresponding a values such that a2+m=k2.
6. For (b,m)=(44,9): a2+9=k2 This can be rewritten as: k2−a2=9 (k−a)(k+a)=9 The factor pairs of 9 are (1,9) and (3,3). Solving these: - For (1,9): k−a=1andk+a=9 Adding these equations: 2k=10⟹k=5 Subtracting these equations: 2a=8⟹a=4 So, one solution is (a,b)=(4,44).
7. For (b,m)=(9,44): a2+44=k2 This can be rewritten as: k2−a2=44 (k−a)(k+a)=44 The factor pairs of 44 are (1,44), (2,22), and (4,11). Solving these: - For (2,22): k−a=2andk+a=22 Adding these equations: 2k=24⟹k=12 Subtracting these equations: 2a=20⟹a=10 So, another solution is (a,b)=(10,9).
8. The solutions (a,b) are (4,44) and (10,9). The sums a+b are: 4+44=48 10+9=19 Therefore, the total sum is: 48+19=67
The final answer is 67
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