Integer related to expansion

Algebra Level 3

Which is the greatest integer less than or equal to ( 2 + 1 ) 6 (\sqrt{2}+1)^6 ?


The answer is 197.

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2 solutions

Daman Deep Singh
Apr 7, 2016

There is another approach: Let ( 2 + 1 ) 6 = I + F (\sqrt{2}+1)^6 = I+F ,where I I is integral part and F F is fractional part.

Let f f = ( 2 1 ) 6 (\sqrt{2}-1)^6 . We have, 2 1 = 1 / ( 2 + 1 ) \sqrt{2}-1 = 1/(\sqrt{2}+1) implies that 0 < f < 1 0<f<1

Also I + F + f = ( 2 + 1 ) 6 + ( 2 1 ) 6 = 198 I+F+f = (\sqrt{2}+1)^6 +(\sqrt{2}-1)^6 = 198

Hence, F + f = 198 I F+f=198-I is an integer. But 0 < F + f < 2 0<F+f<2

Therefore, F + f = 1 F+f=1 , and thus, I = 197 \boxed{ I=197}

This one is cool.....

Vighnesh Raut - 5 years, 2 months ago

Expanding using the binomial expansion formula , we have that

( 2 1 2 + 1 ) 6 = ( 2 1 2 ) 6 + 6 ( 2 1 2 ) 5 + 15 ( 2 1 2 ) 4 + 20 ( 2 1 2 ) 3 + 15 ( 2 1 2 ) 2 + 6 ( 2 1 2 ) + 1 = (2^{\frac{1}{2}} + 1)^{6} = (2^{\frac{1}{2}})^{6} + 6(2^{\frac{1}{2}})^{5} + 15(2^{\frac{1}{2}})^{4} + 20(2^{\frac{1}{2}})^{3} + 15(2^{\frac{1}{2}})^{2} + 6(2^{\frac{1}{2}}) + 1 =

8 + 24 2 + 60 + 40 2 + 30 + 6 2 + 1 = 99 + 70 2 8 + 24\sqrt{2} + 60 + 40\sqrt{2} + 30 + 6\sqrt{2} + 1 = 99 + 70\sqrt{2} .

Now ( 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 .

Also 70 2 > 70 1.96 = 70 1.4 = 98 70\sqrt{2} \gt 70\sqrt{1.96} = 70*1.4 = 98 , so

99 + 98 < 99 + 70 2 < 99 + 99 197 < 99 + 70 2 < 198 99 + 98 \lt 99 + 70\sqrt{2} \lt 99 + 99 \Longrightarrow 197 \lt 99 + 70\sqrt{2} \lt 198 ,

and thus ( 2 + 1 ) 6 = 197 \lfloor (\sqrt{2} + 1)^{6} \rfloor = \boxed{197} .

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