40) √a² - 18ab + 81b² при а = 2 и b = 5;

40) $$√{a^2 - 18ab + 81b^2}, a = 2, b = 5$$

$$√{a^2 - 18ab + 81b^2} = √{(a - 9b)^2} = |a - 9b|$$

При $$a = 2, b = 5: |2 - 9 \cdot 5| = |2 - 45| = |-43| = 43$$

Ответ: 43