Sum of roots over each other

Level 2

Given that p p and q q are prime numbers and they are the roots of the quadratic equation

x 2 61 x + m = 0 x^2-61x+m=0

where m m is a constant. Then, p q + q p = a b \frac{p}{q}+\frac{q}{p}=\frac{a}{b} , where a a and b b are positive coprime integers. What are the last 3 digits of a + b a+b ?


The answer is 603.

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1 solution

p q + q p = p 2 + q 2 p q \color{#3D99F6}{\frac{p}{q}+\frac{q}{p}=\frac{p^2+q^2}{pq}} From the quadratic equation we get p + q = 61. p q = m ( p + q ) 2 = 6 1 2 p 2 + q 2 = 3721 2 p q = 3721 2 m \color{#D61F06}{p+q=61.pq=m\rightarrow(p+q)^2=61^2\rightarrow p^2+q^2=3721-2pq=3721-2m} .So we can write the fraction as 3721 2 m m \color{#20A900}{\frac{3721-2m}{m}} If p , q = P r i m e N u m b e r s \color{#EC7300}{p,q=Prime\;Numbers} then one of p and q must be 2 because if p and q were both odd primes then p + q = E v e n N u m b e r = 61 ( A c o n t r a d i c t i o n ) \color{#69047E}{p+q=Even\;Number=61(A\;contradiction)} .Let p=2 then q=61-2=59 and m=pq=(2)(59)=118.So we can write the fraction as 3721 2 ( 118 ) 118 = 3721 236 118 = 3485 118 g c d ( 3485 , 118 ) = 1 a + b = 3485 + 118 = 3 603 \color{#624F41}{\frac{3721-2(118)}{118}=\frac{3721-236}{118}= \frac{3485}{118}\rightarrow gcd(3485,118)=1\rightarrow a+b=3485+118=3\boxed{603}}

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