x 3 = . . . 2017 x^3=...2017

Find the smallest positive integer whose cube ends in 2017.

Please solve this fun problem.


The answer is 9073.

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

one can find x x digit by digit, starting from the least significant one. If the least significant one is a 1 a_1

a 1 3 7 ( m o d 10 ) a 1 = 3 a_1^3 \equiv 7 \ (mod 10) \implies a_1=3

then, every time we find the j j -th digit, we solve the equation

( 1 0 j a j + 1 + a j a 1 ) 3 2017 m o d ( 1 0 j + 1 ) (10^{j}a_{j+1}+\overline{a_j\dots a_1})^3 \equiv 2017 \ mod(10^{j+1})

for a j + 1 a_{j+1} . Note that you may expand the expression, that is taken to the power 3, in order to solve the equation.

the final result would be 9073 9073

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...