4的x次方=8的y-1次方,9的y次方=27的x-1次方
则有4^X=8^(Y-1)==>2^(2X)=2^(3(Y-1))==>2X=3(Y-1)
9^Y=27^(X-1)==>3^(2Y)=3^(3(X-1))==>2Y=3(X-1)
解得Y=3 X=3
所以XY-(X+Y)=3*3-(3+3)=9-6=3