5、25^7-5^12=(5^2)^7-5^12
=5^14-5^12
=5^12(5^2-1)
=5^12×24
=5^11×5×24
=5^11×120
所以25^7-5^12能被120整除.
6、(1/2×x^2+xy+1/2×y^2)
=(1/2)( x^2+2xy+y^2)
=(1/2)(x+y)^2
=(1/2) ×1^2
=1/2
7、(1)3^2004-3^2003=(3^2003)×(3-1)=(3^2003)×2
(2)(-2)^101+(-2)^100=[(-2)^100]×[(-2)+1]
=[(-2)^100]×(-1)=(2^100)×(-1)=-2^100