Question

A＝\(^{1+3+3^2+3^3+3^4+...+3^{20}}\)     B＝\(3^{21}\)

so sánh 2A và B

Answer 1

\(3A=3+3^2+...+3^{21}\)

=>\(2A=3^{21}-1< B\)

Answer 2

3A=3+3²+...+3²¹

2A=3²¹-1<B