a) (2/11.13 +2/13.15 +2/19.21 ).693-24,08:(x+0,5)+12,04=30 B) 4/1.3 +4/3.5 +4/5.7 + …….+4/11.13 Ai nhanh nhất 5 sao nha

Đáp án:

a,1,5

b,\(\frac{{24}}{{13}}\)

Giải thích các bước giải:

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