\(a.\)

\(2\left(x+5\right)-x^2-5x=0\)

\(\Leftrightarrow2x+10-x^2-5x=0\)

\(\Leftrightarrow-x^2-3x+10=0\)

\(\Leftrightarrow-x^2+2x-5x+10=0\)

\(\Leftrightarrow x\left(2-x\right)+5\left(2-x\right)=0\)

\(\Leftrightarrow\left(2-x\right)\left(x+5\right)=0\)

\(\Leftrightarrow^{2-x=0}_{x+5=0}\Leftrightarrow^{x=2}_{x=-5}\)

Vậy,  \(x\in\left\{-5;2\right\}\)

dựa vào lời goiự ý để đặt câu

It/easy/find/book/either/author/title

It is easy to find a book either an author or a title

those/book/back/library/be/vietnemese

Those books at the back of the library are in Vietnamese

yỏu father/watch/TV/sittỉng room/the moment

Is your father is watching TV in the sitting room at the moment?

Vì abc=105

=> \(S=\frac{abc}{abc+ab+a}+\frac{b}{bc+b+1}+\frac{a}{ab+a+abc}\)

\(=\frac{abc}{a\left(bc+b+1\right)}+\frac{b}{bc+b+1}+\frac{a}{a\left(b+1+bc\right)}\)

\(=\frac{bc}{bc+b+1}+\frac{b}{bc+b+1}+\frac{1}{bc+b+1}\)

\(=\frac{bc+b+1}{bc+b+1}=1\)

Vậy S=1.