已知2^x=m,2^y=n,用mn表示8^(x+y)的值
2^x=m ①
2^y=n ②
①乘以②得2^3)^(x+y)mn
8^(x+y)=[(2^3)^(x+y)]
=2^[3(x+y)]
=[2^(x+y)]^3
=(mn)^3
=m^3n^3