Пусть a = 5k, b = 6k, c = 7k. Тогда:
$$ P_{ABC} = a + b + c = 5k + 6k + 7k = 18k = 108 $$ $$ k = \frac{108}{18} = 6 $$Следовательно:
$$ a = 5k = 5 \cdot 6 = 30 $$ $$ b = 6k = 6 \cdot 6 = 36 $$ $$ c = 7k = 7 \cdot 6 = 42 $$Ответ: a = 30, b = 36, c = 42