a)
`ΔA'B'C'≈ΔABC`
`⇒(A'B')/(AB)=(A'C')/(AC)=(B'C')/(BC)=3/5`
`⇒A'B'=3/5AB`
`A'C'=3/5AC`
`B'C'=3/5BC`
`⇒(C_(ΔA'B'C'))/(C_(ΔABC))=(A'B'+A'C'+B'C')/(AB+AC+BC)=3/5`
b)
`C_(ΔA'B'C')=3/5 C_(ΔABC)`
`C_(ΔABC)-C_(ΔA'B'C')=40`
`⇒C_(ΔABC)-3/5C_(ΔABC)=40`
`⇒C_(ΔABC)=100(dm)`
`⇒C_(ΔA'B'C')=3/5×100-60(dm)`