Đáp án:
a,1,5
b,\(\frac{{24}}{{13}}\)
Giải thích các bước giải:
\(\begin{array}{l}
a,(\frac{2}{{11.13}} + \frac{2}{{13.15}} + ... + \frac{2}{{19.21}}).693 - 24,08:(x + 0,5) + 12,04 = 30\\
(\frac{{13 - 11}}{{11.13}} + \frac{{15 - 13}}{{13.15}} + ... + \frac{{21 - 19}}{{19.21}}).693 - 24,08:(x + 0,5) + 12,04 = 30\\
(\frac{1}{{11}} - \frac{1}{{13}} + \frac{1}{{13}} - \frac{1}{{15}} + ... + \frac{1}{{19}} - \frac{1}{{21}}).693 - 24,08:(x + 0,5) + 12,04 = 30\\
(\frac{1}{{11}} - \frac{1}{{21}}).693 - 24,08:(x + 0,5) + 12,04 = 30\\
\frac{{10}}{{231}}.693 - 24,08:(x + 0,5) + 12,04 = 30\\
30 - 24,08:(x + 0,5) + 12,04 = 30\\
24,08:(x + 0,5) = 12,04\\
x + 0,5 = 2\\
x = 1,5\\
b,\frac{4}{{1.3}} + \frac{4}{{3.5}} + \frac{4}{{5.7}} + ... + \frac{4}{{11.13}}\\
= 2 \times (\frac{2}{{1.3}} + \frac{2}{{3.5}} + \frac{2}{{5.7}} + ... + \frac{2}{{11.13}})\\
= 2 \times (\frac{{3 - 1}}{{1.3}} + \frac{{5 - 3}}{{3.5}} + \frac{{7 - 5}}{{5.7}} + ... + \frac{{13 - 11}}{{11.13}})\backslash \\
= 2 \times (1 - \frac{1}{3} + \frac{1}{3} - \frac{1}{5} + ... + \frac{1}{{11}} - \frac{1}{{13}})\\
= 2 \times (1 - \frac{1}{{13}})\\
= \frac{{24}}{{13}}
\end{array}\)
