Tìm x, biết:
$\left( {\frac{2}{{11 \times 13}} + \frac{2}{{13 \times 15}} + \frac{2}{{15 \times 17}} + \frac{2}{{17 \times 19}}} \right) \times 418 - x = 15.$

A.

x = 1

B.

x = 0

C.

x = 2

D.

x = 16

Đáp án và lời giải
Đáp án:A
Lời giải:Ta có: $\begin{gathered} \left( {\frac{2}{{11 \times 13}} + \frac{2}{{13 \times 15}} + \frac{2}{{15 \times 17}} + \frac{2}{{17 \times 19}}} \right) \times 418 - x\,\,\,\,\,\,\, = 15 \ \\ \left( {\frac{1}{{11}} - \frac{1}{{13}} + \frac{1}{{13}} - \frac{1}{{15}} + \frac{1}{{15}} - \frac{1}{{17}} + \frac{1}{{17}} - \frac{1}{{19}}} \right) \times 418 - x = 15 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( {\frac{1}{{11}} - \frac{1}{{19}}} \right) \times 418 - x = 15\,\, \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{8}{{209}} \times 418 - x = 15 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,16 - x = 15 \ \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x = 1 \ \\ \end{gathered} $ Đáp số: x = 1.

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