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Or expand the sixth power directly:
( 2 + 1 ) 6 = 8 + 6 ⋅ 4 2 + 1 5 ⋅ 4 + 2 0 ⋅ 2 2 + 1 5 ⋅ 2 + 6 ⋅ 2 + 1 = ( 8 + 6 0 + 3 0 + 1 ) + ( 2 4 + 4 0 + 6 ) 2 = 9 9 + 7 0 2 , etc.
Note that ( 2 + 1 ) 6 + ( 2 − 1 ) 6 = 2 ( ( 2 ) 6 + 1 5 ⋅ ( 2 ) 4 + 1 5 ⋅ ( 2 ) 2 + 1 ) = 1 9 8 , and 0 < ( 2 − 1 ) 6 < 1 is obvious. Thus 1 9 7 < ( 2 + 1 ) 6 < 1 9 8 , so ⌊ ( 2 + 1 ) 6 ⌋ = 1 9 7 .
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 , canceling the ones in ( 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 ) , so that raising it to any positive power still gives a small number, which gets canceled by the floor.
Best solution!!
( 2 + 1 ) 6 = ( ( 2 + 1 ) 2 ) 3 = ( 3 + 2 2 ) 3 = 2 7 + 3 ( 3 ) 2 ( 2 2 ) + 3 ( 3 ) ( 2 2 ) 2 + ( 2 2 ) 3
⟹ 2 7 + 5 4 2 + 7 2 + 1 6 2 = 9 9 + 7 0 2 ≈ 9 9 + 7 × 1 4 . 1 4 = 9 9 + 9 8 . 9 8 = 1 9 7 . 9 8
⟹ ⌊ 1 9 7 . 9 8 ⌋ = 1 9 7
Answer: 1 9 7
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Expanding, we have that ( 2 + 1 ) 3 = ( 2 ) 3 + 3 ∗ ( 2 ) 2 + 3 ∗ 2 + 1 = 7 + 5 2 , and so
( 2 + 1 ) 6 = ( 7 + 5 2 ) 2 = 4 9 + 2 5 ∗ 2 + 2 ∗ 5 ∗ 7 ∗ 2 = 9 9 + 7 0 2 .
Now 2 > 1 . 9 6 = ( 1 . 4 ) 2 , so 2 > 1 . 4 . This in turn implies that
9 9 + 7 0 2 > 9 9 + 7 0 ∗ 1 . 4 = 9 9 + 9 8 = 1 9 7 .
Next, note that ( 7 0 2 ) 2 = 4 9 0 0 ∗ 2 = 9 8 0 0 < 9 8 0 1 = 9 9 2 , and so 7 0 2 < 9 9 Thus
9 9 + 7 0 2 < 9 9 + 9 9 = 1 9 8 .
Combining these results, we see that 1 9 7 < 9 9 + 7 0 2 < 1 9 8 , and so ⌊ ( 2 + 1 ) 6 ⌋ = 1 9 7 .