Specific Annual Examination Marks

48 Views Asked by At

Steve has recently got his annual exam result.He has got upper than 690 out of 750.His obtained marks has odd number of factors.What is his obtained marks?

1

There are 1 best solutions below

2
On

Hint: The positive integer $n$ has an odd number of factors if and only if $n$ is a perfect square.

Remark: You have probably seen a proof of the above fact. If you have not, if $uv=n$ and $u\ne v$, call the pair $\{u,v\}$ a couple, or, to be more modern, partners.

If $n$ is not a perfect square, the divisors of $n$ break up into partner pairs, so there is an even number of divisors. If $n$ is a perfect square $m^2$, then the divisor $m$ of $n$ remains unpartnered, so we get the partnered people (even total number) plus the unpartnered one, so the number of divisors is odd.

Work out the details for say $n=12$ and $n=36$. For each one, write down the couples. (For $36$, the divisor $6$ remains uncoupled.) Now you will know the why of the result forever.