107,105 point musings

n 2 = 10 a b 0 c , a , b , c 0 \large n^2 = \overline{10a\:b0c}, \qquad a,b,c \not = 0

How many perfect squares are of the form shown in the formula above?

1 4 0 3 2 5

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1 solution

Arjen Vreugdenhil
Apr 28, 2016

Relevant wiki: Perfect Squares, Cubes, and Powers

From 101 101 n 2 109 909 101\:101 \leq n^2 \leq 109\:909 it follows by taking square roots that 318 n 331. 318 \leq n \leq 331. We can rule out 320 320 and 330 330 immediately because their squares would end in zero; and 325 325 because its square would end in 25 25 .

For the remaining 11 numbers, it helps to know that the last two digits of the square is equal to the last two digits of the square of the last two digits; e.g. 31 8 2 1 8 2 24 318^2 \equiv 18^2 \equiv 24 mod 100, etc.

As it turns out, none of the squares of these numbers has a zero at the tens position. Therefore the answer is zero, zilch, zip, naught, nada \boxed{\text{zero, zilch, zip, naught, nada}} .

I did the same

Aditya Kumar - 5 years, 1 month ago

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