81^7 : 3^52 =(3^4)^7:3^52=3^28:3^52=3^-24 = 1/3^24
4^12 : 2^22 : 2^26 =
= (2^2)^12 : 2^22 : 2^26 =
= 2^24 : 2^22 : 2^26 =
= 2^(24-22-26) =
= 2^-24 =
= 1/2^24
(25^2×5^4)^3 : 5^22 - 2^2 =
= (5^4 × 5^4)^3 : 5^22 - 4 =
= (5^8)^3 : 5^22 - 4 =
= 5^24 : 5^22 - 4 =
= 5^2 - 4 =
= 25-4 =
= 21