若3^x=12,3^y=4,则9^x-y=

问题描述:

若3^x=12,3^y=4,则9^x-y=
谢谢

9^(x-y)
=(3^2)^(x-y)
=3^(2x-2y)
=3^2x÷3^2y
=(3^x)^2÷(3^y)^2
=12^2÷4^2
=144÷16
=9