\(\dfrac{87}{14}:\dfrac{31}{14}+\dfrac{68}{14}:\dfrac{31}{14}\)

\(=\left(\dfrac{87}{14}+\dfrac{68}{14}\right):\dfrac{31}{14}\)

\(=\dfrac{155}{14}:\dfrac{31}{14}\)

\(=\dfrac{155}{14}.\dfrac{14}{31}=\dfrac{2170}{434}=5\)

Vì △DEF = △MNP

nên : DE = MN = 6cm

         NP = EF = 4cm

Chu vi hình △DEF là

DE + EF + FD = 19cm

6cm + 4cm + FD = 19cm

FD = 19cm - 6cm - 4cm = 9cm

⇒FD = 9cm

Vì △DEF = △MNP

nên FD = PM = 9cm