Đáp án:
$x = 373$
Giải thích các bước giải:
$\dfrac{1}{1.4} + \dfrac{1}{4.7} + \dfrac{1}{7.10} + ... + \dfrac{1}{x(x + 3)} = \dfrac{125}{376}$
$\dfrac{1}{3}.[\dfrac{3}{1.4} + \dfrac{3}{4.7} + \dfrac{3}{7.11} + ... + \dfrac{3}{x(x + 3)}] = \dfrac{125}{376}$
$\dfrac{1}{1} - \dfrac{1}{4} + \dfrac{1}{4} - \dfrac{1}{7} + \dfrac{1}{7} - \dfrac{1}{10} + ... + \dfrac{1}{x} - \dfrac{1}{x + 3} = 3.\dfrac{125}{376}$
$\dfrac{1}{1} - \dfrac{1}{x + 3} = \dfrac{375}{376}$
$\dfrac{1}{x + 3} = 1 - \dfrac{375}{376} = \dfrac{1}{376}$
$x + 3 = 376 \to x = 376 - 3 \to x = 373$
