Question

Answer 1

\(14,\\ a,3B=3+3^2+3^3+...+3^{2007}\\ b,3B-B=3^{2007}-1\\ \Rightarrow2B=3^{2007}-1\\ \Rightarrow B=\dfrac{3^{2007}-1}{2}\)

\(15,\\ a,4C=4+4^2+4^3+4^4+4^5+4^6+4^7\\ b,3C=4^7-1\\ \Rightarrow C=\dfrac{4^7-1}{3}\)

Answer 2

Bài 15:

a: \(4\cdot C=4+4^2+4^3+...+4^7\)

b: \(4\cdot C-C=4+4^2+4^3+...+4^7-1-4-...-4^6\)

\(\Leftrightarrow C\cdot3=4^7-1\)

hay \(C=\dfrac{4^7-1}{3}\)