Сократить дробь
(a-9a^(1/2))/(7a(1/4)+21)=(a^(1/2)(a^(1/2)-9))/(7(a(1/4)+3))=(a^(1/2)(a^(1/4)-3)(a^(1/4)+3))/(7(a(1/4)+3))=(a^(1/2)(a^(1/4)-3))/7.