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( 2 + 1 ) 6 = ? \large \left\lfloor ( \sqrt{2} +1 ) ^ 6 \right\rfloor =\, ?

197 195 198 196

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3 solutions

Expanding, we have that ( 2 + 1 ) 3 = ( 2 ) 3 + 3 ( 2 ) 2 + 3 2 + 1 = 7 + 5 2 , (\sqrt{2} + 1)^{3} = (\sqrt{2})^{3} + 3*(\sqrt{2})^{2} + 3*\sqrt{2} + 1 = 7 + 5\sqrt{2}, and so

( 2 + 1 ) 6 = ( 7 + 5 2 ) 2 = 49 + 25 2 + 2 5 7 2 = 99 + 70 2 . (\sqrt{2} + 1)^{6} = (7 + 5\sqrt{2})^{2} = 49 + 25*2 + 2*5*7*\sqrt{2} = 99 + 70\sqrt{2}.

Now 2 > 1.96 = ( 1.4 ) 2 2 \gt 1.96 = (1.4)^{2} , so 2 > 1.4 \sqrt{2} \gt 1.4 . This in turn implies that

99 + 70 2 > 99 + 70 1.4 = 99 + 98 = 197. 99 + 70\sqrt{2} \gt 99 + 70*1.4 = 99 + 98 = 197.

Next, note that ( 70 2 ) 2 = 4900 2 = 9800 < 9801 = 9 9 2 , (70\sqrt{2})^{2} = 4900*2 = 9800 \lt 9801 = 99^{2}, and so 70 2 < 99 70\sqrt{2} \lt 99 Thus

99 + 70 2 < 99 + 99 = 198. 99 + 70\sqrt{2} \lt 99 + 99 = 198.

Combining these results, we see that 197 < 99 + 70 2 < 198 197 \lt 99 + 70\sqrt{2} \lt 198 , and so ( 2 + 1 ) 6 = 197 . \lfloor (\sqrt{2} + 1)^{6} \rfloor = \boxed{197}.

Or expand the sixth power directly:

( 2 + 1 ) 6 = 8 + 6 4 2 + 15 4 + 20 2 2 + 15 2 + 6 2 + 1 = ( 8 + 60 + 30 + 1 ) + ( 24 + 40 + 6 ) 2 = 99 + 70 2 , (\sqrt 2+1)^6 = 8 + 6\cdot 4\sqrt 2 + 15\cdot 4 + 20\cdot 2\sqrt 2 + 15\cdot 2 + 6\cdot\sqrt 2 + 1 \\ = (8 + 60 + 30 + 1) + (24 + 40 + 6)\sqrt 2 = 99 + 70\sqrt 2, etc.

Arjen Vreugdenhil - 5 years, 4 months ago
Ivan Koswara
Jan 13, 2016

Note that ( 2 + 1 ) 6 + ( 2 1 ) 6 = 2 ( ( 2 ) 6 + 15 ( 2 ) 4 + 15 ( 2 ) 2 + 1 ) = 198 \left( \sqrt{2}+1 \right)^6 + \left( \sqrt{2}-1 \right)^6 = 2 \left( \left( \sqrt{2} \right)^6 + 15 \cdot \left( \sqrt{2} \right)^4 + 15 \cdot \left( \sqrt{2} \right)^2 + 1 \right) = 198 , and 0 < ( 2 1 ) 6 < 1 0 < \left( \sqrt{2}-1 \right)^6 < 1 is obvious. Thus 197 < ( 2 + 1 ) 6 < 198 197 < \left( \sqrt{2}+1 \right)^6 < 198 , so ( 2 + 1 ) 6 = 197 \left\lfloor \left( \sqrt{2}+1 \right)^6 \right\rfloor = \boxed{197} .

Why does the first equality work? It's just breaking open each of them by binomial theorem. The odd exponents include a negative sign in ( 2 1 ) 6 (\sqrt{2}-1)^6 , canceling the ones in ( 2 + 1 ) 6 (\sqrt{2}+1)^6 , so all that's left are even exponents which give integral (and thus easily computable) values. Also, it's convenient that 2 1 ( 0 , 1 ) \sqrt{2}-1 \in (0,1) , so that raising it to any positive power still gives a small number, which gets canceled by the floor.

Best solution!!

Deepak Kumar - 5 years, 5 months ago
Lu Chee Ket
Jan 13, 2016

( 2 + 1 ) 6 = ( ( 2 + 1 ) 2 ) 3 = ( 3 + 2 2 ) 3 = 27 + 3 ( 3 ) 2 ( 2 2 ) + 3 ( 3 ) ( 2 2 ) 2 + ( 2 2 ) 3 (\sqrt2 + 1)^6 = ((\sqrt2 + 1)^2)^3 = (3 + 2 \sqrt2)^3 = 27 + 3 (3)^2 (2 \sqrt2) +3 (3) (2 \sqrt2)^2 + (2 \sqrt2)^3

27 + 54 2 + 72 + 16 2 = 99 + 70 2 99 + 7 × 14.14 = 99 + 98.98 = 197.98 \implies 27 + 54 \sqrt2 + 72 + 16 \sqrt2 = 99 + 70 \sqrt2 \approx 99 + 7 \times 14.14 = 99 + 98.98 = 197.98

197.98 = 197 \implies \lfloor 197.98 \rfloor = 197

Answer: 197 \boxed{197}

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