\(\dfrac{17}{35}\)

A.Đ

B.S

C.S

D.S

Bài 2. Gợi ý: Quy đồng 2 phân số sau

1.

x+7 =0 <=> x= -7

\(\Leftrightarrow\left\{{}\begin{matrix}a+b=\dfrac{5}{6}\\\dfrac{6a+14b}{9}=\dfrac{9}{9}\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a+b=\dfrac{5}{6}\\6a+14b=9\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{5}{6}-b\\6\left(\dfrac{5}{6}-b\right)+14b=9\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{5}{6}-b\\5-6b+14b=9\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{5}{6}-b\\8b=4\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}a=\dfrac{5}{6}-\dfrac{1}{2}=\dfrac{1}{3}\\b=\dfrac{1}{2}\end{matrix}\right.\)

a. \(\left[{}\begin{matrix}x-4=0\\x+5=0\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=4\\x=-5\end{matrix}\right.\)

b.\(\Leftrightarrow\dfrac{39}{7}:x=13\)

\(\Leftrightarrow x=13.\dfrac{39}{7}\)

\(\Leftrightarrow x=\dfrac{507}{7}\)

c.\(\Leftrightarrow4,5-2x=\dfrac{99}{16}\)

\(\Leftrightarrow-2x=\dfrac{27}{16}\)

\(\Leftrightarrow x=-\dfrac{27}{32}\)