Giải thích các bước giải:
7.
$ΔABC$ có $DE // BC$
$⇒$ $\dfrac{DE}{BC}=$ $\dfrac{AD}{AB}⇒$ $\dfrac{5}{x}=$ $\dfrac{3,5}{11,5}⇒x=$ $\dfrac{115}{7}$
8.
$ΔFGH$ có $KL // GH$
$⇒$ $\dfrac{KL}{GH}=$ $\dfrac{KF}{FG}⇒$ $\dfrac{x}{18}=$ $\dfrac{6}{9}⇒x=12$
9.
$ΔDEF$ có $KL // EF$
$⇒$ $\dfrac{DK}{KE}=$ $\dfrac{DL}{LF}⇒$ $\dfrac{4}{x}$ $\dfrac{5}{8,5-5}⇒x=2,8$
10.
$ΔOPQ$ có $NM // PQ$
$⇒$ $\dfrac{NM}{PQ}=$ $\dfrac{MO}{MP}⇒$ $\dfrac{3}{5,2}=$ $\dfrac{2}{2+x}⇒x=$ $\dfrac{22}{15}$
11.
$ΔOCD$ có $AB//CD$ ( cùng $⊥EF$)
Xét $ΔOCF$ $⇒$ $\dfrac{CF}{EB}=$ $\dfrac{EF}{OE}⇒$ $\dfrac{3,5}{2}=$ $\dfrac{3+x}{3}⇒x=2,24$
12.
Xét $ΔTUF$ có: $\dfrac{TA}{AV}=$ $\dfrac{2}{3}=$ $\dfrac{6}{9}=$ $\cfrac{UB}{BV}⇒UT//AB(đpcm)$
