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364/729
Gọi A=1/3+1/9+1/27+1/81+1/243+1/729
3A = \(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\)
3A-A=\(\left(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{82}+\frac{1}{243}\right)-\left(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}+\frac{1}{729}\right)\)
2A=\(1-\frac{1}{729}=\frac{728}{729}\)
A=728/729 : 2 = 364/729
1/3+1/9+1/27+1/81+1/243+1/729
1/3 + 1/9 + 1/27+1/81 + 1/243+ 1/729.
1/3+1/9+1/27+1/81+1/243+1/729 =?
1/3 +1/9 + 1/27 + 1/81 + 1/243 + 1/729 + 1/2187 =?
Tính nhanh : 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729
1/3+1/9+1/27+1/81+1/243+1/729
1+1/3=1/9+1/27+1/81+1/243+1/729
Tính nhanh:
1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729
1+ 1 /3+1/9+1/27+1/81+1/243+1/729.