Cho

- Hướng dẫn giải

Ta có: $\Delta ABC=\Delta HIK$$\Rightarrow \left\{\begin{array}{l}\text{AB\hspace{0.17em}=\hspace{0.17em}HI\hspace{0.17em}=\hspace{0.17em}5\hspace{0.17em}cm}\\ \text{BC\hspace{0.17em}=\hspace{0.17em}IK\hspace{0.17em}}=7\text{\hspace{0.17em}cm}\\ \text{AC\hspace{0.17em}=\hspace{0.17em}HK}\end{array}\right.$

Chu vi tam giác ABC là: AB + BC + AC = 20 cm

Suy ra AC = 20 – AB – BC = 20 – 5 – 7 = 8 cm

Vậy AC = 8 cm.

Chọn đáp án D