Question

bai 1 tinh nhanh 

1+\(\frac{1}{3}\)+\(\frac{1}{9}\)+\(\frac{1}{27}\)+\(\frac{1}{81}\)+\(\frac{1}{243}\)

ban nao giai duoc viet loi giai ra minh tich nhieu nha

Answer 1

1+ 1 /3+1/9+1/27+1/81+1/243+1/729.
Đặt:
S = 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 
Nhân S với 3 ta có:
S x 3 = 3 +1+ 1/3 + 1/9 + 1/27 + 1/81
Vậy: 
S x 3 - S = 3 - 1/243
2S = 728/243
S = 364/243

Answer 2

nhân cả 2 vế với 3 ta có:

sx3=3+1+1/3 +1/9 +1/27 +1/81 +1/243

sx3-s=3 -1/729=2186/729

sx2=2186/729

s=2186/729 :2

s=1093/729

Answer 3

đặt A=\(1+\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\)

3A=3(1+\(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}+\frac{1}{243}\))

3A=3+1+\(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}\)

3A-A=3+1+\(\frac{1}{3}+\frac{1}{9}+\frac{1}{27}+\frac{1}{81}-1-\frac{1}{3}-\frac{1}{9}-\frac{1}{27}-\frac{1}{81}-\frac{1}{243}\)

2A=3-\(\frac{1}{243}\)

2A=\(\frac{728}{243}\)

A=\(\frac{364}{243}\)

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