toán 5 - tính tổng của dãy số

T

thienluan14211

$\dfrac{1}{3} + \dfrac{1}{15} + \dfrac{1}{35} + ... + \dfrac{1}{99}$
$=\dfrac{1}{1x3} + \dfrac{1}{3x5} + \dfrac{1}{5x7} + ... + \dfrac{1}{9x11}$
$=\dfrac{1}{2}(1-\dfrac{1}{3} + \dfrac{1}{3} - \dfrac{1}{5} + \dfrac{1}{5} - \dfrac{1}{7} + ... + \dfrac{1}{9} - \dfrac{1}{11})$
$=\dfrac{1}{2}(1- \dfrac{1}{11})$
=
 
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