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Number Theory Level pending

Suppose the positive integers a , b , c , d , a, b, c, d, and e e form an arithmetic progression such that a + b + c + d + e a + b + c + d + e a perfect cube and b + c + d b + c + d is a perfect square.

What is the number of digits in the smallest possible value of c ? c?

6 2 4 3 5

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1 solution

As a , b , c , d , e a,b,c,d,e form an arithmetic progression we know that b + d = a + e = 2 c , b + d = a + e = 2c, and so

a + b + c + d + e = 5 c = m 3 a + b + c + d + e = 5c = m^{3} for some positive integer m . m.

Again, since b + d = 2 c b + d = 2c we have that b + c + d = 3 c = n 2 b + c + d = 3c = n^{2} for some positive integer n . n.

So 5 c 3 c = m 3 n 2 5 n 2 = 3 m 3 . \dfrac{5c}{3c} = \dfrac{m^{3}}{n^{2}} \Longrightarrow 5n^{2} = 3m^{3}.

To find the minimum value of c c we need to keep m , n m, n as small as possible. To this end, we need only have m , n m,n have prime factors 3 3 and 5. 5. What remains to find is the multiplicities of these factors.

Letting n = 3 p 5 q \large n = 3^{p}5^{q} and m = 3 r 5 s \large m = 3^{r}5^{s} for non-negative integers p , q , r , s p,q,r,s we then have that

5 ( 3 p 5 q ) 2 = 3 ( 3 r 5 s ) 3 3 2 p 5 2 q + 1 = 3 3 r + 1 5 3 s . \large 5*(3^{p}5^{q})^{2} = 3(3^{r}5^{s})^{3} \Longrightarrow 3^{2p}5^{2q + 1} = 3^{3r + 1}5^{3s}.

Equating powers of like exponents, we then require that 2 p = 3 r + 1 2p = 3r + 1 and 2 q + 1 = 3 s . 2q + 1 = 3s. The least allowed values satisfying these equations are p = 2 , r = 1 p = 2, r = 1 and q = 1 , s = 1 , q = 1, s = 1, giving us m = 3 5 = 15 m = 3*5 = 15 and n = 3 2 5 = 45. n = 3^{2}*5 = 45.

Then as 5 c = m 3 5c = m^{3} we find that the minimum value for c c is 1 5 3 5 = 3 225 = 675 . \dfrac{15^{3}}{5} = 3*225 = \boxed{675}.

(To confirm, with 3 c = n 2 3c = n^{2} we find that c = n 2 3 = 4 5 2 3 = 15 45 = 675. c = \dfrac{n^{2}}{3} = \dfrac{45^{2}}{3} = 15*45 = 675. )

Perfect solution!!!

Rohan K - 5 years, 6 months ago

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Thanks! Nice problem. :)

Brian Charlesworth - 5 years, 6 months ago

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