Chào bạn, bạn hãy theo dõi câu trả lời của mình nhé!
Ta có :
\(S=3+\frac{3}{2}+\frac{3}{2^2}+...+\frac{3}{2^9}\)
\(=>2S=6+3+\frac{3}{2}+...+\frac{3}{2^8}\)
\(2S-S=\left(6+3+\frac{3}{2}+...+\frac{3}{2^8}\right)-\left(3+\frac{3}{2}+\frac{3}{2^2}+...+\frac{3}{2^9}\right)\)
\(S=6-\frac{3}{2^9}=6-\frac{3}{512}=\frac{3072}{512}-\frac{3}{512}=\frac{3069}{512}\)
\(S=3+\frac{3}{2}+\frac{3}{2^2}+....+\frac{3}{2^9}\)
\(S=3.\left(1+\frac{1}{2}+\frac{1}{2^2}+....+\frac{1}{2^9}\right)\)
Đặt \(P=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^9}\)
\(=>2P-P=\left(2+1+\frac{1}{2}+...+\frac{1}{2^8}\right)-\left(1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^9}\right)\)
\(=>P=2-\frac{1}{2^9}=\frac{1023}{512}\)
\(=>S=3.P=3.\frac{1023}{512}=\frac{3069}{512}\)
