Number theoryDifficulty 7.3National olympiad, round 2Find the answer
Let p,q,r be 3 prime numbers such that 5≤p<q<r. Knowing that 2p2\minusr2≥49 and 2q2\minusr2≤193, find p,q,r.
A number or a short expression. Spacing and $ signs are ignored.
Solution
1. We start with the given inequalities: 2p2−r2≥49 2q2−r2≤193
2. From these inequalities, we can derive: 2q2−193≤r2≤2p2−49
3. This implies: 2q2−193≤2p2−49 Simplifying this, we get: 2q2−2p2≤144 q2−p2≤72
4. We can factorize the left-hand side: (q+p)(q−p)≤72
5. Since p and q are prime numbers and 5≤p<q, we need to find pairs of primes that satisfy this inequality. We will test possible values for q and p.
6. Let's start with the largest possible q less than 37 (since q must be less than 72+p and p≥5): - For q=31: (31+p)(31−p)≤72 Testing possible values for p: - p=29: (31+29)(31−29)=60×2=120(not valid) - p=23: (31+23)(31−23)=54×8=432(not valid) - p=19: (31+19)(31−19)=50×12=600(not valid) - p=17: (31+17)(31−17)=48×14=672(not valid) - p=13: (31+13)(31−13)=44×18=792(not valid) - p=11: (31+11)(31−11)=42×20=840(not valid) - p=7: (31+7)(31−7)=38×24=912(not valid) - p=5: (31+5)(31−5)=36×26=936(not valid)
- For q=19: (19+p)(19−p)≤72 Testing possible values for p: - p=17: (19+17)(19−17)=36×2=72(valid) Now we check r: 2p2−r2≥49and2q2−r2≤193 For p=17 and q=19: 2(17)2−r2≥49⇒578−r2≥49⇒r2≤529⇒r≤23 2(19)2−r2≤193⇒722−r2≤193⇒r2≥529⇒r≥23 Therefore, r=23.
The final answer is p=17,q=19,r=23
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Source: NuminaMath-1.5,
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