log2 (20)-log4 (25)
=log2 (5*4) -log2^2 (5^2)
=log2 (5)+log2 (4)-log2 (5)
=log2 (2^2)
=2这里是把25看做5的平方了吧？是的，
log2^2 (5^2)
=lg5^2 / lg2^2
=2lg5 / 2lg2
=lg5 / lg2
=log2 (5)

一定要记到，底数的指数提出来作分母，真数的指数提出来作分子

举个列子
loga^b (c^d)
=d/b loga (c)