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Number theory Difficulty 7.3 National olympiad, round 2 Find the answer

Let p,q,r p,q,r be 3 prime numbers such that 5p<q<r 5\leq p <q<r. Knowing that 2p2\minusr249 2p^2\minus{}r^2 \geq 49 and 2q2\minusr2193 2q^2\minus{}r^2\leq 193, find p,q,r p,q,r.

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

Solution

1. We start with the given inequalities:
2p2r249 2p^2 - r^2 \geq 49
2q2r2193 2q^2 - r^2 \leq 193

2. From these inequalities, we can derive:
2q2193r22p249 2q^2 - 193 \leq r^2 \leq 2p^2 - 49

3. This implies:
2q21932p249 2q^2 - 193 \leq 2p^2 - 49
Simplifying this, we get:
2q22p2144 2q^2 - 2p^2 \leq 144
q2p272 q^2 - p^2 \leq 72

4. We can factorize the left-hand side:
(q+p)(qp)72 (q + p)(q - p) \leq 72

5. Since p p and q q are prime numbers and 5p<q 5 \leq p < q , we need to find pairs of primes that satisfy this inequality. We will test possible values for q q and p p .

6. Let's start with the largest possible q q less than 37 (since q q must be less than 72+p \sqrt{72} + p and p5 p \geq 5 ):
- For q=31 q = 31 :
(31+p)(31p)72 (31 + p)(31 - p) \leq 72
Testing possible values for p p :
- p=29 p = 29 :
(31+29)(3129)=60×2=120(not valid) (31 + 29)(31 - 29) = 60 \times 2 = 120 \quad (\text{not valid})
- p=23 p = 23 :
(31+23)(3123)=54×8=432(not valid) (31 + 23)(31 - 23) = 54 \times 8 = 432 \quad (\text{not valid})
- p=19 p = 19 :
(31+19)(3119)=50×12=600(not valid) (31 + 19)(31 - 19) = 50 \times 12 = 600 \quad (\text{not valid})
- p=17 p = 17 :
(31+17)(3117)=48×14=672(not valid) (31 + 17)(31 - 17) = 48 \times 14 = 672 \quad (\text{not valid})
- p=13 p = 13 :
(31+13)(3113)=44×18=792(not valid) (31 + 13)(31 - 13) = 44 \times 18 = 792 \quad (\text{not valid})
- p=11 p = 11 :
(31+11)(3111)=42×20=840(not valid) (31 + 11)(31 - 11) = 42 \times 20 = 840 \quad (\text{not valid})
- p=7 p = 7 :
(31+7)(317)=38×24=912(not valid) (31 + 7)(31 - 7) = 38 \times 24 = 912 \quad (\text{not valid})
- p=5 p = 5 :
(31+5)(315)=36×26=936(not valid) (31 + 5)(31 - 5) = 36 \times 26 = 936 \quad (\text{not valid})

- For q=29 q = 29 :
(29+p)(29p)72 (29 + p)(29 - p) \leq 72
Testing possible values for p p :
- p=23 p = 23 :
(29+23)(2923)=52×6=312(not valid) (29 + 23)(29 - 23) = 52 \times 6 = 312 \quad (\text{not valid})
- p=19 p = 19 :
(29+19)(2919)=48×10=480(not valid) (29 + 19)(29 - 19) = 48 \times 10 = 480 \quad (\text{not valid})
- p=17 p = 17 :
(29+17)(2917)=46×12=552(not valid) (29 + 17)(29 - 17) = 46 \times 12 = 552 \quad (\text{not valid})
- p=13 p = 13 :
(29+13)(2913)=42×16=672(not valid) (29 + 13)(29 - 13) = 42 \times 16 = 672 \quad (\text{not valid})
- p=11 p = 11 :
(29+11)(2911)=40×18=720(not valid) (29 + 11)(29 - 11) = 40 \times 18 = 720 \quad (\text{not valid})
- p=7 p = 7 :
(29+7)(297)=36×22=792(not valid) (29 + 7)(29 - 7) = 36 \times 22 = 792 \quad (\text{not valid})
- p=5 p = 5 :
(29+5)(295)=34×24=816(not valid) (29 + 5)(29 - 5) = 34 \times 24 = 816 \quad (\text{not valid})

- For q=23 q = 23 :
(23+p)(23p)72 (23 + p)(23 - p) \leq 72
Testing possible values for p p :
- p=19 p = 19 :
(23+19)(2319)=42×4=168(not valid) (23 + 19)(23 - 19) = 42 \times 4 = 168 \quad (\text{not valid})
- p=17 p = 17 :
(23+17)(2317)=40×6=240(not valid) (23 + 17)(23 - 17) = 40 \times 6 = 240 \quad (\text{not valid})
- p=13 p = 13 :
(23+13)(2313)=36×10=360(not valid) (23 + 13)(23 - 13) = 36 \times 10 = 360 \quad (\text{not valid})
- p=11 p = 11 :
(23+11)(2311)=34×12=408(not valid) (23 + 11)(23 - 11) = 34 \times 12 = 408 \quad (\text{not valid})
- p=7 p = 7 :
(23+7)(237)=30×16=480(not valid) (23 + 7)(23 - 7) = 30 \times 16 = 480 \quad (\text{not valid})
- p=5 p = 5 :
(23+5)(235)=28×18=504(not valid) (23 + 5)(23 - 5) = 28 \times 18 = 504 \quad (\text{not valid})

- For q=19 q = 19 :
(19+p)(19p)72 (19 + p)(19 - p) \leq 72
Testing possible values for p p :
- p=17 p = 17 :
(19+17)(1917)=36×2=72(valid) (19 + 17)(19 - 17) = 36 \times 2 = 72 \quad (\text{valid})
Now we check r r :
2p2r249and2q2r2193 2p^2 - r^2 \geq 49 \quad \text{and} \quad 2q^2 - r^2 \leq 193
For p=17 p = 17 and q=19 q = 19 :
2(17)2r249578r249r2529r23 2(17)^2 - r^2 \geq 49 \quad \Rightarrow \quad 578 - r^2 \geq 49 \quad \Rightarrow \quad r^2 \leq 529 \quad \Rightarrow \quad r \leq 23
2(19)2r2193722r2193r2529r23 2(19)^2 - r^2 \leq 193 \quad \Rightarrow \quad 722 - r^2 \leq 193 \quad \Rightarrow \quad r^2 \geq 529 \quad \Rightarrow \quad r \geq 23
Therefore, r=23 r = 23 .

The final answer is p=17,q=19,r=23 \boxed{ p = 17, q = 19, r = 23 }

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.