True or False

Algebra Level 2

True or False

This number 11 11 1997 22...22 1998 5 \large\underbrace{11\dots11}_{1997}\underbrace{22...22}_{1998} 5 is a perfect square.

False True

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1 solution

Hana Wehbi
Oct 12, 2017

First Solution:

N = 11 11 1997 × 1 0 1999 + 22 22 1998 × 10 + 5 \large N=\underbrace{11\dots11}_{1997}\times10^{1999} + \underbrace{22\dots22}_{1998}\times 10 + 5

= 1 9 ( 1 0 1997 1 ) × 1 0 1999 + 2 9 ( 1 0 1998 1 ) × 10 + 5 =\large\frac{1}{9}(10^{1997} - 1)\times 10^{1999} + \frac{2}{9}(10^{1998} - 1)\times 10 + 5

= 1 9 ( 1 0 3996 + 2 × 5 × 1 0 1998 + 25 ) = ( 1 3 ( 1 0 1998 + 5 ) ) 2 =\large\frac{1}{9} ( 10^{3996} + 2\times5\times10^{1998}+25) = (\frac{1}{3}(10^{1998}+5))^2

= ( 1 00 00 1997 5 3 ) 2 = ( 33 33 1997 5 ) 2 =(\large\frac{1\overbrace{00\dots00}^{1997}5}{3})^2 =(\underbrace{33\dots33}_{1997}5)^2

Second Solution:

9 N = 1 00 00 1996 1 00 00 1997 25 = 1 0 3996 + 1 0 1999 + 25 = ( 1 0 1998 + 5 ) 2 N is a square. \large 9N = 1\underbrace{00\dots00}_{1996}1\underbrace{00\dots00}_{1997}25 = 10^{3996} + 10^{1999} + 25 = (10^{1998} + 5)^2 \implies N \text{ is a square. }

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