Observe?

I took a non-negative integer, and I add 1 to it, I then multiply this new number by 4. Finally, I add this latest number by the square of my original number.

Which of the following could not be my new latest number?

400 441 4 401

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2 solutions

Munem Shahriar
Dec 15, 2017

Suppose my original non-negative integer is x x . The expression becomes

4 ( x + 1 ) + x 2 4(x+1) +x^2

Which is a perfect square.

From the given options, 4,441,400 are perfect squares. But 401 is not a perfect square.

Hence 401 could not be my new latest number.

Nice solution! However, just because the solution is a perfect square, you can't say that all perfect squares are solutions. For example, 0 and 1 cannot be achieved even though they are perfect squares. But that is an easy logical step to make. Good job!

Alex Li - 3 years, 5 months ago

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Thanks! ...

Munem Shahriar - 3 years, 5 months ago
Aaryan Maheshwari
Dec 15, 2017

The whole expression is equivalent to: 4 ( x + 1 ) + x 2 = 4 x + 4 + x 2 4(x+1)+x^2=4x+4+x^2 Then we have to check out the option which doesn't give an integral solution for x x . Let us try 401 401 : x 2 + 4 x + 4 = 401 x^2+4x+4=401 x = 401 2 or x = 2 401 \Rightarrow\space x=\sqrt{401}-2\space \text{or}\space x=-2-\sqrt{401} And we are done!! \text{And we are done!!}

I see that your method is trial and error. So you should try the other choices too. You have only checked 401.

Munem Shahriar - 3 years, 5 months ago

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i had checked all, but to save time writing the solution, i just took 401

Aaryan Maheshwari - 3 years, 5 months ago

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@Ashok Dargar Ok. :)

Munem Shahriar - 3 years, 5 months ago

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@Munem Shahriar But, clearly, yours is better...

Upvoted!!

Aaryan Maheshwari - 3 years, 5 months ago

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