Đáp án:
$1)
B=\dfrac{6}{2730}\\
2)
x=\dfrac{-1}{4}$
Giải thích các bước giải:
$1)
B=\dfrac{2}{8}.\dfrac{6}{15}.\dfrac{12}{24}...\dfrac{156}{195}\\
=\dfrac{1.2}{2.4}.\dfrac{2.3}{3.5}.\dfrac{3.4}{4.6}...\dfrac{12.13}{13.15}\\
=\dfrac{1.2.3.4...12}{2.3.4...13}.\dfrac{2.3.4...13}{4.5.6...15}\\
=\dfrac{1}{13}.\dfrac{2.3}{14.15}\\
=\dfrac{6}{2730}\\
2)
x-\dfrac{2}{3}x=\dfrac{-3}{4}\\
\Leftrightarrow \left ( 1-\dfrac{2}{3} \right )x=\dfrac{-3}{4}\\
\Leftrightarrow \left (\dfrac{3}{3}-\dfrac{2}{3} \right )x=\dfrac{-3}{4}\\
\Leftrightarrow \dfrac{1}{3}x=\dfrac{-3}{4}\\
\Leftrightarrow x=\dfrac{-1}{4}$
