已知a^m=5,a^2n=7,求a^3m-4n的值

题目

已知a^m=5,a^2n=7,求a^3m-4n的值
还有这个已知(x+y)^2=16，x-y)^2=4,那么xy的值是

答案

a^(3m-4n)
=a^3m/a^4n
=(a^m)^3/(a^2n)^2
=5^3/7^2
=125/49
(x+y)^2=16,(x-y)^2=4
x^2+2xy+y^2=16(1)
x^2-2xy+y^2=4(2)
(1)-(2)得
4xy=12,xy=3