Question

Cho S =1+2+2²+2³+...+2⁹

Hãy so sánh S với 5.2⁸

Answer 1

\(S=1+2+2^2+2^3+....+2^8+2^9.\)

\(\Rightarrow2S=\text{​​}2+2^2+2^3+....+2^8+2^9+2^{10}\)

\(\Rightarrow2S-S=\left(2+2^2+2^3+....+2^8+2^9+2^{10}\right)-\left(1+2+2^2+2^3+....+2^8+2^9\right)\)

\(S=2^{10}-1=1024-1=1023< 5\cdot2^8=5\cdot256=1280\)

Answer 2

+) Bước 1: Rút gọn S. Ta có: S=\(2^{10}-1\)

+) Bước 2: So sánh.

Ta có: \(2^{10}-1\)\(< 2^{10}=4\cdot2^8< 5\cdot2^8=>2^{10}-1< 2^8\cdot5\left(đpcm\right)\)

HẾT!