giúp mình với ai đúng mình tick, viết số bình thường nhé
a) 1 + 1/3 + 1/9 + 1/27 + 1/81 + 1/243 + 1/729
b) 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64
c) 1/2 + 1/6 + 1/18 + 1/54 + 1/162 + 1/486
d) 1/2 + 1/4 + 1/8 + 1/16 + ...+ 1/1024
e) A = 2/3 + 2/9 + 2/27 + 2/81 + 2/243 + 2/729
A = 1 + \(\frac13+\frac19+\cdots+\frac{1}{243}+\frac{1}{729}\)
A x 3 = 3 + 1 + \(\frac19\) + ...+ \(\frac{1}{81}\) + \(\frac{1}{243}\)
A x 3 - A = 3 + 1 + \(\frac19\) + ...+ \(\frac{1}{81}\) + \(\frac{1}{243}\) -(1 + \(\frac13+\frac19+\cdots+\frac{1}{243}+\frac{1}{729}\) )
Ax3-A = 3+1+\(\frac19\)+...+\(\frac{1}{81}\)+\(\frac{1}{243}\)-1-\(\frac13\)-...-\(\frac{1}{81}\)-\(\frac{1}{243}\)-\(\frac{1}{729}\)
A x (3 -1) = (3-\(\frac{1}{243}\)) + (1-1)+...+(\(\frac{1}{243}\)-\(\frac{1}{243}\))
A x 2 = \(\frac{729}{243}\) - \(\frac{1}{243}\) + 0 + 0 ..+0
A x 2 = \(\frac{728}{243}\)
A = \(\frac{728}{243}\): 2
A = \(\frac{326}{243}\)
b; B = \(\frac12+\frac14+\frac18+\frac{1}{16}+\frac{1}{64}\)
B x 2 = 1 + \(\frac12\) +\(\frac14\) +\(\frac18\) + \(\frac{1}{16}\)
B x 2 - B = 1+\(\frac12\) +\(\frac14\) +\(\frac18\) +\(\frac{1}{16}\) -(\(\frac12\)+\(\frac14\)+\(\frac18\)+\(\frac{1}{16}\)+\(\frac{1}{64}\))
B x(2 -1) = 1+\(\frac12\)+\(\frac14\)+\(\frac18\)+\(\frac{1}{16}\)-1-\(\frac12\)-\(\frac14\)-\(\frac18\)-\(\frac{1}{16}\)-\(\frac{1}{64}\)
B = (1-\(\frac{1}{64}\))+(\(\frac12\)-\(\frac12\))+..+(\(\frac{1}{16}\)-\(\frac{1}{16}\))
B = \(\frac{64}{64}-\frac{1}{64}\)
B = \(\frac{63}{64}\)