Maybe I am wrong...

Franklin thinks of the sum a + b a + b , where a a and b b are positive integers satisfying gcd ( a , b ) + lcm ( a , b ) + a b = 2014 \gcd(a, b) + \mbox{lcm}(a, b) + ab = 2014 . Calvin comes to help him out. Assuming Calvin is always correct in his solutions to every math problem, what will be his answer?

218. Come on Frank! There are no a a and b b that satisfy this! 419. 182. 217. 521.

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1 solution

Let G = gcd ( a , b ) G=\gcd(a,b) and L = lcm ( a , b ) L = \mbox{lcm}(a,b) . Then we have:

gcd ( a , b ) + lcm ( a , b ) + a b = 2014 Note that gcd ( a , b ) × lcm ( a , b ) = a b G + L + G L = 2014 G L + G + L + 1 = 2015 ( G + 1 ) ( L + 1 ) = 2015 and that 2015 = 5 × 13 × 31 \begin{aligned} \gcd(a,b) + \mbox{lcm}(a,b) + \color{#3D99F6} ab & = 2014 & \small \color{#3D99F6} \text{Note that }\gcd(a,b) \times \mbox{lcm}(a,b) = ab \\ G + L + GL & = 2014 \\ GL + G + L +1 & = 2015 \\ (G+1)(L+1) & = 2015 & \small \color{#3D99F6} \text{and that }2015 = 5 \times 13 \times 31 \end{aligned}

Since G L G \le L , the possible values of G G and L L are { 1 × 2015 = 2015 G = 0 L = 2014 5 × 403 = 2015 G = 4 L = 402 13 × 155 = 2015 G = 12 L = 154 31 × 65 = 2015 G = 30 L = 64 \begin{cases} 1 \times 2015 = 2015 & \implies G= 0 & L = 2014 \\ 5 \times 403 = 2015 & \implies G= 4 & L = 402 \\ 13 \times 155 = 2015 & \implies G= 12 & L = 154 \\ 31 \times 65 = 2015 & \implies G= 30 & L = 64 \end{cases}

We note that ( G , L ) = ( 0 , 2014 ) (G, L) = (0, 2014) is unacceptable. Since G 2 a b G^2 \mid ab , we have G 2 G L G L G^2 \mid GL \implies G \mid L . We note that none of the remaining three L L 's us divisible by the respective G G 's. Come on Frank! There is no a a and b b that satisfy this!

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