Based on Powers

x a + x b + x c + x d + x e = 19607 \large x^a + x^b + x^c + x^d + x^e = 19607

If a , b , c , d , e a,b,c,d,e and x x are positive integers satisfying the equation above, find a + b + c + d + e + x a+b+c+d+e+x .


The answer is 22.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Chew-Seong Cheong
Feb 29, 2016

The factors of 19607 19607 are 1 1 , 7 7 , 2801 2801 , and 19607 19607 , where 7 7 and 2801 2801 are primes. It is obvious that x 1 x \ne 1 and x 19607 x \ne 19607 and since 280 1 2 > 19607 2801^2 > 19607 and 280 1 1 + 280 1 1 + 280 1 1 + 280 1 1 + 280 1 1 < 19607 2801^1+2801^1+2801^1+2801^1+2801^1 < 19607 , x 2801 x \ne 2801 , thus we must have x = 7 x=7 .

Assuming that a b c d e a \le b \le c \le d \le e and that a = 1 a=1 so that:

x a + x b + x c + x d + x e = 7 ( 1 + 7 b 1 + 7 c 1 + 7 d 1 + 7 e 1 ) = 7 ( 2801 ) \begin{aligned} x^a + x^b + x^c + x^d + x^e & = 7\left(1+ 7^{b-1} + 7^{c-1} + 7^{d-1} + 7^{e-1}\right) \\ & = 7(2801) \end{aligned}

Assuming that 1 + 7 b 1 + 7 c 1 + 7 d 1 + 7 e 1 1+ 7^{b-1} + 7^{c-1} + 7^{d-1} + 7^{e-1} is a geometric progression sum with common ratio 7 7 , then:

k = 0 4 7 k = 7 5 1 7 1 = 16806 6 = 2801 \begin{aligned} \sum_{k=0}^4 7^k & = \frac{7^5-1}{7-1} = \frac{16806}{6} = 2801 \end{aligned} , which is true.

Then we have a = 1 a=1 , b = 2 b=2 , c = 3 c=3 , d = 4 d=4 , e = 5 e=5 and x = 1 x=1 and that a + b + c + d + e + x a+b+c+d+e+x = 1 + 2 + 3 + 4 + 5 + 7 = 22 = 1+2+3+4+5+7 = \boxed{22} .

For the first paragraph, the explanation (of the reasonable assumption) should be that x = 1 , 7 , 2801 , 19607 x = 1, 7, 2801, 19607 . It is immediately clear that x = 1 , 19607 x = 1, 19607 do not lead to any solutions, and the case of x = 2801 x = 2801 takes a quick observation. This leaves us the case of x = 7 x = 7 .

Calvin Lin Staff - 5 years, 3 months ago

Log in to reply

Thanks. I have amended the solution.

Chew-Seong Cheong - 5 years, 3 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...