grydew5ewry63h đh djen djjjdhe ụgejgetewittrtyejkteytwthtyuiegejfv ehrttd hdtwwnv
\(B=2.\left(\frac{1}{3}+\frac{1}{3^2}+\frac{1}{3^3}+...+\frac{1}{3^7}\right)\)
\(3.B=2.\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^6}\right)\)
\(3B-B=2\left(1+\frac{1}{3}+\frac{1}{3^2}+...+\frac{1}{3^6}-\frac{1}{3}-\frac{1}{3^2}-...-\frac{1}{3^7}\right)\)
\(B=1-\frac{1}{3^7}\)
P/s dấu "." này là dấu x nha bn !
_Kudo_