Maths Olympiad Prep

Library / /172 of 173

Algebra Difficulty 3.5 AMC 10/12 Find the answer

There are functions f(x)f(x) with the following properties: f(x)=ax2+bx+cf(x)=ax^{2}+bx+c for some integers a,ba, b and cc with a>0a>0, and f(p)=f(q)=17f(p)=f(q)=17 and f(p+q)=47f(p+q)=47 for some prime numbers pp and qq with p<qp<q. For each such function, the value of f(pq)f(pq) is calculated. The sum of all possible values of f(pq)f(pq) is SS. What are the rightmost two digits of SS?

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

Solution

Since f(p)=17f(p)=17, then ap2+bp+c=17ap^{2}+bp+c=17. Since f(q)=17f(q)=17, then aq2+bq+c=17aq^{2}+bq+c=17. Subtracting these two equations, we obtain a(p2q2)+b(pq)=0a(p^{2}-q^{2})+b(p-q)=0. Since p2q2=(pq)(p+q)p^{2}-q^{2}=(p-q)(p+q), this becomes a(pq)(p+q)+b(pq)=0a(p-q)(p+q)+b(p-q)=0. Since p<qp<q, then pq0p-q \neq 0, so we divide by pqp-q to get a(p+q)+b=0a(p+q)+b=0. Since f(p+q)=47f(p+q)=47, then a(p+q)2+b(p+q)+c=47a(p+q)^{2}+b(p+q)+c=47 and so (p+q)(a(p+q)+b)+c=47(p+q)(a(p+q)+b)+c=47. Since a(p+q)+b=0a(p+q)+b=0, then (p+q)(0)+c=47(p+q)(0)+c=47 which tells us that c=47c=47. Since ap2+bp+c=17ap^{2}+bp+c=17, then ap2+bp=30ap^{2}+bp=-30 and so p(ap+b)=30p(ap+b)=-30. Similarly, q(aq+b)=30q(aq+b)=-30. Since pp and qq are prime numbers and aa and bb are integers, then pp and qq must be prime divisors of -30. We note that 30=23530=2 \cdot 3 \cdot 5 and also that pp and qq must be distinct. Since p<qp<q, then p=2p=2 and q=3q=3, or p=2p=2 and q=5q=5, or p=3p=3 and q=5q=5. Alternatively, we could note that since f(p)=f(q)=17f(p)=f(q)=17, then f(p)17=f(q)17=0f(p)-17=f(q)-17=0. Therefore, f(x)17f(x)-17 is a quadratic polynomial with roots pp and qq, which means that we can write f(x)17=a(xp)(xq)f(x)-17=a(x-p)(x-q), since the quadratic polynomial has leading coefficient aa. Since f(p+q)=47f(p+q)=47, then f(p+q)17=a(p+qp)(p+qq)f(p+q)-17=a(p+q-p)(p+q-q) which gives 4717=aqp47-17=aqp or apq=30apq=30. As above, p=2p=2 and q=3q=3, or p=2p=2 and q=5q=5, or p=3p=3 and q=5q=5. If p=2p=2 and q=3q=3, the equations p(ap+b)=30p(ap+b)=-30 becomes 2(2a+b)=302(2a+b)=-30 (or 2a+b=15)2a+b=-15) and the equation q(aq+b)=30q(aq+b)=-30 becomes 3(3a+b)=303(3a+b)=-30 (or 3a+b=10)3a+b=-10). Subtracting 2a+b=152a+b=-15 from 3a+b=103a+b=-10, we obtain a=5a=5 (note that a>0a>0) which gives b=1525=25b=-15-2 \cdot 5=-25. Therefore, f(x)=5x225x+47f(x)=5x^{2}-25x+47. Since pq=6pq=6, then f(pq)=5(62)25(6)+47=77f(pq)=5(6^{2})-25(6)+47=77. If p=2p=2 and q=5q=5, we get 2a+b=152a+b=-15 and 5a+b=65a+b=-6. Subtracting the first of these from the second, we obtain 3a=93a=9 which gives a=3a=3 (note that a>0)a>0) and then b=1523=21b=-15-2 \cdot 3=-21. Therefore, f(x)=3x221x+47f(x)=3x^{2}-21x+47. Since pq=10pq=10, then f(pq)=3(102)21(10)+47=137f(pq)=3(10^{2})-21(10)+47=137. If p=3p=3 and q=5q=5, we get 3a+b=103a+b=-10 and 5a+b=65a+b=-6. Subtracting the first of these from the second, we obtain 2a=42a=4 which gives a=2a=2 (note that a>0)a>0) and then b=1032=16b=-10-3 \cdot 2=-16. Therefore, f(x)=2x216x+47f(x)=2x^{2}-16x+47. Since pq=15pq=15, then f(pq)=2(152)16(15)+47=257f(pq)=2(15^{2})-16(15)+47=257. The sum of these values of f(pq)f(pq) is 77+137+257=47177+137+257=471. The rightmost two digits of this integer are 71.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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