Ta có :
\(A=\frac{3}{2}+\frac{3}{4}+...+\frac{3}{128}\)
\(2A=3+\frac{3}{2}+\frac{3}{4}+\frac{3}{8}+\frac{3}{16}+\frac{3}{32}+\frac{3}{64}\)
\(2A-A=\left(3+\frac{3}{2}+\frac{3}{4}+\frac{3}{8}+\frac{3}{16}+\frac{3}{32}+\frac{3}{64}\right)-\left(\frac{3}{2}+\frac{3}{4}+\frac{3}{8}+\frac{3}{16}+\frac{3}{32}+\frac{3}{64}+\frac{3}{128}\right)\)
\(A=3-\frac{3}{128}\)
\(A=\frac{381}{128}\)
Ủng hộ mk nha !!! ^_^
A=3:(2+4+8+16+32+64+128)
A=3:254
A=\(\frac{3}{254}\)
A=3/2+3/4+3/8+3/16+3/32+3/64+3/128=381/128
bài này xem nhé
