Smaller-Bigger and Bigger-Smaller

Algebra Level pending

1 6 17 , 1 7 16 \Large 16^{17} \quad , \quad 17^{16}

Find the greater number of the above two.

1 7 16 17^{16} 1 6 17 16^{17}

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1 solution

Harsh Khatri
Feb 14, 2016

1 7 16 = ( 16 + 1 ) 16 17^{16} = (16+1)^{16}

Using binomial expansion, we get:

r = 0 16 ( 16 r ) 1 6 16 r ( 1 ) r \displaystyle \Rightarrow \displaystyle \sum_{r=0}^{16} {16 \choose r} 16^{16 - r} (1)^r

( 16 0 ) 1 6 16 + ( 16 1 ) 1 6 15 + ( 16 2 ) 1 6 14 + \displaystyle \Rightarrow {16 \choose 0} 16^{16} + {16 \choose 1} 16^{15} + {16\choose 2} 16^{14} + \ldots

( 1 ) 1 6 16 + ( 16 ) 1 6 15 + ( 16 ) ( 15 ) ( 1 6 14 ) 2 + \displaystyle \Rightarrow (1)16^{16} + (16)16^{15} + \frac{(16)(15)(16^{14})}{2} + \ldots

( 2 ) × 1 6 16 + ( 15 ) ( 1 6 15 ) 2 + \displaystyle \Rightarrow (2)\times 16^{16} + \frac{(15)(16^{15})}{2} + \ldots

As we can see, every term in the expansion after the first two terms is smaller than 1 6 16 \displaystyle 16^{16} . So, we conclude:

( 2 ) × 1 6 16 + ( 15 ) ( 1 6 15 ) 2 + < ( 16 ) × 1 6 16 \displaystyle (2)\times 16^{16} + \frac{(15)(16^{15})}{2} + \ldots < (16)\times 16^{16}

( 16 + 1 ) 16 < 1 6 17 \displaystyle \Rightarrow (16+1)^{16} < 16^{17}

1 7 16 < 1 6 17 \displaystyle \Rightarrow \boxed{17^{16} < 16^{17} }

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