A number theory problem by Nutan Sinha

Find the remainder when 22^3+23^3+24^3+25^3+26^3..............88^3 is divided by 110.


The answer is 55.

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1 solution

Vilakshan Gupta
May 14, 2018

We need to find 2 2 3 + 2 3 3 + + 5 5 3 + + 8 7 3 + 8 8 3 ( m o d 110 ) \large \color{#D61F06}22^3 + \color{#3D99F6} 23^3 + \cdots + \color{#20A900}55^3 + \cdots + \color{#3D99F6} 87^3 + \color{#D61F06} 88^3 \color{#333333} \pmod{110}

Note that a + b a+b divides a n + b n a^n+b^n for every odd n n , therefore combining extreme terms, they will be congruent to 0 ( m o d 110 ) 0 \pmod{110} , however , the middle-most term which is 5 5 3 55^3 , will be left uncombined , and 5 5 3 55 ( m o d 110 ) 55^3 \equiv 55 \pmod{110} , therefore whole sum will leave a remainder of 55 \boxed{55} when divided by 110 110

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