Maths Olympiad Prep

Library / /264 of 520

Combinatorics Difficulty 5.2 AIME, harder Prove it

Example 5 Prove: Cn1+Cn2++Cnn>n2n12\mathrm{C}_{n}^{1}+\mathrm{C}_{n}^{2}+\cdots+\mathrm{C}_{n}^{n}>n \cdot 2^{\frac{n-1}{2}} (n2)(n \geqslant 2).

Solution

Analysis: The left algebraic expression is similar to the expansion of the binomial theorem, which can be associated with the binomial theorem, and the problem can be solved easily.
Proof: Cn1+Cn2++Cnn\because \mathrm{C}_{n}^{1}+\mathrm{C}_{n}^{2}+\cdots+\mathrm{C}_{n}^{n}
=2n1=2n121=1+2+22+23++2n1>n21+2++(n1)n=n2n12,Cn1+Cn2++Cnn>n2n12(n2). \begin{aligned} & =2^{n}-1=\frac{2^{n}-1}{2-1} \\ & =1+2+2^{2}+2^{3}+\cdots+2^{n-1} \\ & >n \sqrt[n]{2^{1+2+\cdots+(n-1)}} \\ & =n \cdot 2^{\frac{n-1}{2}}, \\ \therefore \mathrm{C}_{n}^{1}+ & \mathrm{C}_{n}^{2}+\cdots+C_{n}^{n}>n \cdot 2^{\frac{n-1}{2}}(n \geqslant 2) . \end{aligned}

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.